Electro-optic system for crosswind measurement

ABSTRACT

An electro-optic system, e.g., mounted to a weapon, measures down range winds and a range-to-target for compensating the ballistic hit point. The system may include an optical light source, collimated to generate a laser spot on the target. The system may include a wind measurement receiver that captures laser light scattered from the target. The captured light may be modulated by atmospheric scintillation eddies, producing optical patterns which change in time and move with the crosswind. These patterns may be analyzed by a processor using covariance techniques in either the time-domain or the frequency-domain to determine path-integrated crosswinds and associated errors. Ranging is done by measuring the time of flight of the laser pulse to the target collecting the scattered signal from the target. Compensated ballistic hit point, measurement errors and other data may be displayed on a micro-display digital eyepiece, or projected onto the direct view optics (DVO) of a riflescope so as to be overlaid in real-time on the optical image of the target.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 14/450,076 filed on Aug. 1, 2014 as a continuation-in-part ofU.S. patent application Ser. No. 14/140,163, filed on Dec. 24, 2013, thecontents of both of the aforementioned applications being incorporatedherein by reference in their entireties.

FIELD OF THE INVENTION

The present invention is directed to a system and method for measuringdownrange path integrated winds and a range-to-target for purposes ofcompensating a ballistic hit point.

BACKGROUND OF THE INVENTION

Deer hunting sport has been practiced for many centuries. Bettermaterials, better weapon designs and ammunitions allow hunters to engagetargets at longer ranges and with more precision than was possible inthe past. Typical rifles such as Remington® model 243 allow engagementsto ranges greater than 400 meters. However, it has been recognized thatthere are a number of factors that affect the position of the projectilehit point. Two dominant sources of hit point errors are the uncertaintyin the estimation of the magnitude and direction of path integratedcrosswind and the range to target. The effect of these errors on the hitpoint grows substantially as the standoff range between the hunter andthe target increases and/or the crosswind increases. FIG. 1 shows theeffect of uncompensated average crosswind and imprecise rangemeasurement on hit probability for a 12 inch diameter target, as afunction of range. The graph shows that for an average downrangecrosswind of approximately 5 miles per hour (mph) and a 10% rangeuncertainty, the 243 caliber bullet misses the target aim point at 400meters by over 13 cm. The effect is much worse at longer ranges, forinstance, the bullet misses the target by 31 cm at a range of 600meters, while missing the target by 63 cm at a range of 800 meters.Because a hunter cannot easily and accurately estimate the average windand range to the target, there is a reduced probability of a first roundtarget hit. During the day, an experienced user can estimate thecrosswinds by viewing the mirage through the riflescope or thevegetation motion and the range to target by comparing the target sizeto the crosshair reticle but is unable to conduct these functions duringthe night or in twilight. Improved methods implemented include ananemometer at the hunter's location to estimate local winds and a laserrange finder to estimate the range. If wind and range estimates wereavailable, a ballistics calculator may then be used to calculate thewind and vertical hold offset coordinates. Even these advancements areinadequate. The anemometers just measure the local winds, while handheldrange finders are difficult to keep on target, providing inaccurateresults. Downrange winds can be significantly different than localwinds; they can be non-uniform and change direction and/or amplitudealong the flight path. These changes can be due to causes such asnon-uniform terrain channeling and environmental pressure or temperaturedifferential changes. For example, the effect of downrange winds on thehit point may be negligible if the crosswinds of same amplitude areblowing in one direction for one part of the path and in the oppositedirection for another part of the path cancelling the overall projectiledeviation. Because the local wind sensor cannot measure downrange winds,it provides an offset that would lead to a target miss.

Recent art, as disclosed in US Patent Application Publication No.2013/0206836 A1, teaches the use of various forms of internal orexternal wind sensors at the user's position; all of which measure localwinds. The assumption made in the previous art is that the downrangecrosswinds are the same as measured by the local wind sensor.Experienced users know that this assumption is inaccurate because theprojectile in flight integrates the winds as it flies along itstrajectory to the target.

US Patent Application Publication No. 2013/0206836 A1 teaches the optionof using LIDAR or laser Doppler Anemometry (or velocimetry) for windmeasurement. The LIDAR method cannot easily measure projectile pathcrosswinds unless measurements are made in three known off-axialdirections and the path-average crosswind calculated from the vectoraddition. This means that the measurement is not made close to the paththe projectile travels. In addition the system requires impracticallaser powers to achieve high accuracy at even modest ranges because theback-scattered signal modulated from aerosols in the atmosphere isapproximately 6 orders of magnitude smaller than a modulated signalscattered from a solid target surface. Clear days, with high visibilityto 23 km, can further reduce the range of engagement. This imposesstringent demands on required laser power, laser current drivers, powersupply and signal processing, making the system size too big forpractical mounting on the weapon. The Laser Doppler Anemometry approachto measuring winds involves detecting the scatter from particulatespassing through a small volume generated at the intersection of twointerfering laser beams. It is therefore a point measurement, and doesnot provide path-integrated wind from the shooter to the target.

Downrange path-integrated crosswind measurements from the shooter to thetarget are necessary to accurately predict the hit point of aprojectile. Because the opportunity to engage and hit the target is timesensitive, all measurements must be done in near real time to calculateand display the offset aim point (OAP) in the user's sight; otherwisethe opportunity may be permanently lost.

Other prior art, as disclosed in U.S. Pat. No. 8,196,828, proposes tomeasure downrange integrated crosswind using a laser collimated beam,single aperture and a single imager. In this approach, a high speedcamera is used to image the laser spot on the target with a frame ratehigh enough to freeze the motion of the time varying scintillationpattern. The outbound laser beam is modulated by the atmosphericturbulence producing a time varying pattern of light and dark spots onthe target that move and change with the wind. By measuring the time-lagcovariance of geometrically-related pixel pairs in a series of recordedcamera frames, the path-averaged crosswind can be calculated. Thisapproach suffers from several drawbacks, including: 1) the effect of thereturn path turbulence on the signal scattered from the target acts as anoise source reducing the overall signal to noise ratio; 2) because theability to resolve the light and dark spots on the target is limited bydiffraction of the collecting lens aperture, large lens apertures (inexcess of 100 mm) are required thus increasing the size of the system;3) higher laser signal power is required because the returned signalspreads over many pixels due to aperture diffraction requiring higheroptical power per pixel to measure the crosswind, thus significantlyaffecting battery life; 4) the approach is sensitive to the refractiveindex structure constant, Cn² which reduces the size of the dark andlight spots at values exceeding 10⁻¹³, requiring even higher opticalresolution (i.e., an aperture larger than 100 mm and more opticalpower).

Other prior art, U.S. Pat. No. 8,279,287 and U.S. Patent ApplicationPublication No. 2010/0128136, propose to measure downrange pathintegrated crosswind using a passive method. The technique uses at leasttwo apertures with each aperture passively imaging the target withoutactive light illumination. The atmospheric turbulence modulates theimage of the target which appears wavy due to low-frequency wind motion.Using block matching processing approach, the transit time difference inthe waviness of a single or multiple features from the two images of thetarget is measured to deduce the path-integrated crosswind. The approachrequires multiple high contrast features on the target or sharp targetedge that must first be identified using an imaging sensor and thenprocessed to measure the time difference. Uniform targets withoutfeatures or that blend into the background (camouflaged) cannot beresolved easily. To resolve the target features (approximately 1 cm) at1 km, diffraction limited lens diameter of approximately 150 mm atvisible wavelengths is required. The size of two such lenses makes thedevice impractical for mounting on a weapon.

Another approach is described in the article by Wang et al., “Windmeasurements by the temporal cross-correlation of the opticalscintillations,” Applied Optics V20, No. 23, December 1981. This articledescribes a breadboard system for measuring the path averaged crosswindconfigured such that a laser source at one end transmits light throughthe atmospheric turbulence and is detected by a pair of side by sideoptical receivers located at the other end. This one-way transmissionsystem method can measure path-integrated average crosswinds usingseveral processing techniques. All of these processing techniques arebased on observing the wind-driven motion of the scintillation patternthat transits across the line of sight. For the hunting application, theone-way transmission system is clearly impractical because the laser andoptical receivers must both be on the same side (user's end) of thepath.

When adapting this one-way transmission system to a two-way reflectivesystem, one of the key problems encountered is the laser speckle noisegenerated from the illuminated target. Laser speckle is an interferenceeffect that creates non-uniform distribution of the light intensity(light and dark spots) when laser light reflects back from a targetsurface that has a surface roughness smaller than the coherence lengthof the laser. The speckle problem does not exist in the one-way systembecause light does not scatter from a target. In the two-way case, thelaser light is scattered from the target and collected by the receiverslocated near the light source. Speckles generated at the target andreflected back appear similar to the scintillation pattern signal, whichis created by atmospheric turbulence and used for measuring winds. As aresult the covariance function is disturbed by the interference fromspeckle effects causing large errors in the wind measurement. To addressthis problem, a laser source with a short coherence length, compared tothe target roughness, is required.

Because the aforementioned article by Wang et al. described a fieldexperiment, the system disclosed therein did not have any size, weightand power constraints to meet. Any practical weapon mounted device,demands a compact size that can be operated for extended periods on onebattery charge. As the diameter of the receiver lens decreases to allowa more compact system package, the received signal level goes down andaperture diffraction spreads the focused image over a larger area (ahigher number of pixels if a camera receiver is used) which results inreduced SNR per pixel even if the total energy over all pixels issummed. This limits the size of the receiver lens that can be used. Inthe same way, if a laser divergence of 100 micro radians is required toensure that a laser spot appears on the target at maximum range, thediffraction limits the minimum achievable lens diameter at thatwavelength. Large transmitter and receiver apertures impose sizeconstraints in designing a weapon mounted or portable compact systempackage.

In designing a compact system to measure a path-weighted averagecrosswind and a range-to-target, it would be advantageous to provide theuser with an offset aim point (OAP) indicator in the sight thatconsiders the second order effects from other variables such as:temperature, pressure, humidity, rifle-cant and tilt, ammunition type,etc. Sensors to measure these parameters should be small enough to notimpact the size of the package significantly. Furthermore the packagemust be rugged enough to withstand the shock from repeated weaponfirings. These constraints impose yet more challenges in the innovationof a small and portable system useful for operation on or off a weapon.

SUMMARY OF THE INVENTION

According to an exemplary embodiment, the present invention is directedto a portable system including an optical transmitter, an opticalreceiver, a display device, and a processor. The processor is programmedto receive signals from the optical receiver that are representative oflight transmitted by the optical transmitter and scattered back from atarget; process the received signals to determine a path-weightedaverage crosswind between the transmitter and the target, and a range tothe target; and calculate, based on the path-weighted average crosswindand the range, an offset aim point for display on the system device.This system may be mounted to a weapon such as a rifle, gun, archer'sbow, or crossbow or may be used as a standalone system to measurepath-weighted average crosswinds.

The optical transmitter may be configured to produce a collimated beamthat may create a small light spot on the target. Also, the opticalreceiver may be configured to include first and second wind measurementchannels separated from one another in a horizontal direction or in avertical direction by a predetermined distance to measure the horizontalor vertical component of the crosswind. These first and second windmeasurement channels may be used for detecting light reflected from thetarget which travels back toward the system through atmosphericturbulence eddies which modulates the light and move with crosswinds.Particularly, an image sensor may detect such light variations via saidfirst and second wind measurement channels, respectively, at ameasurable time difference. Since this time difference is dependent onthe downrange crosswind speeds (as well as the predetermined distanceseparating the first and second wind measurement channels), the systemis able to determine a path-weighted average crosswind in the horizontaldirection by the measurement of transit time difference.

The aforementioned first and second wind measurement channels maycomprise separate apertures and optical paths, or alternatively, mayshare a single aperture and optical path. In the latter case, an imagethat is obtained via the single aperture/optical path can be dividedinto two halves, each half corresponding to a respective time-varyingsignal, and the time difference between these signals may be calculatedin order to measure the crosswind speed.

Furthermore, the processor may obtain repeated measurements of theaforementioned transit time difference based on the detection results ofthe first and second wind measurement channels. Based on these repeatedmeasurements, the processor can calculate respective crosswind valuesand average them together to obtain the path-weighted average crosswind.Furthermore, the processor can calculate a confidence metricrepresenting the accuracy of the determined path-weighted averagecrosswind based on a variance of the repeated measurements.

In addition, the processor may apply one or more weighting functionswhich are applied to the detection results of the first and second windmeasurement channels, such weighting functions representingcontributions from respective segments of a downrange crosswind.Multiple weighting functions may be applied by changing the divergenceof the laser beam from the optical transmitter, or else by extractinglaser beam spots of different angular sizes from image data captured bythe wind measurement channels.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given herein below and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention. A brief description of these drawings is asfollows.

FIG. 1 is a diagram illustrating effects of uncompensated crosswinds andimprecise range measurement on the hit probability.

FIG. 2 is a block diagram of an electro-optic system for measuringcrosswind, according to an exemplary embodiment of the presentinvention.

FIG. 3 is a block diagram of components of a laser system implemented inan electro-optic system for measuring crosswind, according to anexemplary embodiment of the present invention.

FIG. 4 is a block diagram of components of a ranging receiverimplemented in an electro-optic system for measuring crosswind,according to an exemplary embodiment of the present invention.

FIG. 5 and FIG. 6 are block diagrams of respective wind measurementchannels implemented in an electro-optic system for measuring crosswind,according to an exemplary embodiment of the present invention.

FIG. 7 is a block diagram of a digital riflescope utilizing principlesof the present invention, according to an exemplary embodiment.

FIG. 8 and FIG. 9 illustrate respective external views of implementationof a rifle-mounted electro-optic system for measuring crosswind,according to an exemplary embodiment of the present invention.

FIG. 10 is a flowchart illustrating a process of data collection for useby an electro-optic system in combination with range and crosswindmeasurements, according to an exemplary embodiment of the presentinvention.

FIG. 11 is a flowchart illustrating an algorithm for performing asequence of measurements using an electro-optic system for measuringcrosswind, according to an exemplary embodiment of the presentinvention.

FIG. 12 is a flowchart illustrating an algorithm for obtaining a singlemeasurement of a path-weighted averaged crosswind, according to anexemplary embodiment of the present invention.

FIG. 13 is a block diagram illustrating various sensors used in additionto the wind and range measurements by a processor to calculate theballistics offset aim-point, according to an exemplary embodiment of thepresent invention.

FIG. 14 is a block diagram illustrating various components in a wirelessinput/output (I/O) module for receiving input data (e.g., ammunition andweapon type) wirelessly, and also for wirelessly transmitting data(e.g., video clips, images and performance data), according to anexemplary embodiment of the present invention.

FIG. 15 shows examples of four different weighting functions, which areproduced by varying the laser divergence, for use by a processor indetermining and combining multiple path-weighted average crosswindsaccording to an exemplary embodiment.

FIG. 16 shows examples of four different weighting functions, which arederived from image data by software, for use by a processor indetermining and combining multiple path-weighted average crosswindsaccording to an exemplary embodiment.

FIG. 17 illustrates examples of respective signals arriving at first andsecond wind measurement channels with a time difference, according to anexemplary embodiment of the present invention.

FIG. 18 illustrates a correspondence between measurements repeatedlycollected over a period of time (e.g., while the user continues to pressa switch) and repeated covariance calculations, which are used to updatea confidence metric, according to an exemplary embodiment of the presentinvention.

FIG. 19 illustrates an example of the target image overlaid with theoffset aim point (OAP), confidence metric indicator arrows, electroniccrosshair and other calculated parameters, according to an exemplaryembodiment of the present invention.

FIG. 20 is a block diagram of an electro-optic system for measuringcrosswind, according to an alternative embodiment.

FIG. 21 is a flowchart illustrating a process which may be applied inthe alternative embodiment to determine crosswinds based on a frequencyof scintillation fade.

FIG. 22 is a flowchart illustrating a process which may be applied inthe alternative embodiment to calculate the scintillation fade frequencyfrom the detection results of a wind measurement channel.

FIG. 23 is a flowchart illustrating a process which may be applied inthe alternative embodiment to determine the crosswind direction based onthe detection results of a wind measurement channel.

FIGS. 24A, 24B & 24C illustrate examples of respective stages of atime-varying scintillation signal during a process of calculating thescintillation fade frequency in the alternative embodiment.

FIG. 25 illustrates an example of a data collection during a fieldexperiment for calculating a calibration constant for use in determiningthe scintillation fade frequency in connection with the alternativeembodiment.

FIG. 26 is a block diagram of an electro-optic system for measuringcrosswind, according to a further embodiment for projecting symbologyonto the target image of an existing riflescope.

FIG. 27 is a block diagram of a symbology projector which may be used inthe further embodiment.

FIG. 28 is a simplified illustration of the use of two optical aperturesto measure crosswind by calculating a time delay between the twotime-varying signals received via the two apertures.

FIG. 29 is a graph correlating changes in a refractive index structureconstant to changes in the measured laser spot size.

FIG. 30 is a diagram illustrating the configuration of a single opticalaperture to measure crosswind by calculating a time delay between twotime-varying signals received via the single aperture, according to asecond alternative embodiment.

FIG. 31 is a diagram illustrating a particular optical arrangement foradapting the single aperture structure of FIG. 30 to provide dualoptical paths for pupil plane imaging (for measuring crosswind) and forfield imaging (for boresighting and measuring laser spot size),according to the second alternative embodiment.

FIG. 32A and FIG. 32B illustrate respective views of a layout of opticalreceivers which can be implemented as part of an electro-optic systemfor measuring crosswind according to the second alternative embodiment.

FIG. 33 is a flowchart illustrating an enhanced processing algorithm formeasuring the crosswind by compensating for environmental factors.

DETAILED DESCRIPTION

An exemplary embodiment of a system, which is referred to hereinafter asthe “XeroWind System” (or “XWS” for short), is designed for use as areplacement to the riflescope to not only measure the crosswind andrange but also to provide an image of the target for acquisition andaiming. According to one alternate embodiment, however, the XWS may beconfigured as a clip-on so that the users may retain their direct-viewriflescopes if desired. In another alternate embodiment, the XWS may beused as a standalone system mounted to a tripod for assisting a spotter.The system can measure the path-weighted average crosswind between theuser and the target, the range to the target, and then, using theammunition characteristics (mass, size, shape, muzzle velocity),atmospheric data and weapon angles, and other data, it calculates theazimuth and elevation offset aim points (OAP) to compensate for the winddeflection and ballistic drop of the ammunition used. The calculatedoffsets and the confidence metric may be displayed as icons overlaid inthe target image which is used by the hunter to re-position the weaponaim-point. When the confidence icons have converged to approximately thesize of the target, this can alert the hunter to fire the weapon. If theconfidence icons are close to the target size and not changing rapidly,the user can be confident that the measurement will be valid forsufficient time to engage the target. The system may also provide fordownloading the static target image or a short video clip onto a cellphone or other computing device.

FIG. 2 is a functional block diagram showing the major subsystems of arifle-mounted XWS 100. The XWS 100 employs a laser transmitter 300 as anoptical source and two optical receivers 500, 600 that are located in arugged housing (which is shown in FIG. 8 and FIG. 9). The housing may beclamped to the weapon using a mounting point (e.g., element 110 of FIG.9) allowing the system to measure path integrated crosswind and range toa target when requested by the user via, e.g., a push button switch 1000positioned near the trigger at convenient location. Using a measuredcrosswind value and a range value to the target, as well as otherpossible measurements which will be described later (e.g.,meteorological data, projectile characteristics, weapon orientation,location, and motion), the XWS 100 then calculates an offset aim pointin the azimuth and elevation directions and displays these offsets inreal time to the user in a micro-display 1100, which is viewed through aset of lenses configured into a magnifying eyepiece. Due to variableconditions such as temporary downrange wind gusts, the accuracy of thewind measurement can vary with time and conditions. The system thereforemay also calculate wind measurement errors that are displayed to theuser as a “confidence metric” letting the shooter know when it is a goodtime to engage or wait for better shooting condition (FIG. 19illustrates an example of displaying a confidence metric through the useof arrows 5004 and 5005).

The laser system 300 includes a semiconductor diode and collimationoptics to generate a narrow beam on the order of 50 micro radians. Itmay also provide the illumination for imaging the target area during thenight, e.g., when a holographic disperser is used in its path to producea second larger divergence beam (on the order of 44 milli-radians). The50 micro radian narrow laser beam is used for measuring range and windusing a ranging receiver 400 to detect and analyze the reflected returnswhen the laser is in the ranging mode, and two wind measurement channels500 and 600, which are optical receivers with panchromatic image sensorsthat detect the laser returns when in the wind measurement mode. Onewind measurement channel 600 may also image the target area on ahigh-resolution color image sensor 120 (also shown in FIG. 6), andprovide the image signal to the micro-display 1100. A number of localsensors 800 (which, as shown in FIG. 13, may include: environmental ormeteorological (met) sensors 802, 803, and 804, GPS 807, digitalmagnetic compass (DMC) 801, accelerometers 805 and gyroscopes 806) canbe used to supply data used by the ballistic calculator. WirelessInput/Output system 900, potential components of which are shown infurther detail in FIG. 14, may allow the user to input data for theammunition and rifle type. Push button switches 1000 allow the user toselect the mode of system operation, color micro-display 1100 providesthe user with a high resolution image of the target area overlaid with adigitally generated crosshair and the aim point offsets, non-volatilememory 1200 holds the processing and control software. Thecontroller/processor 1300 provides the signal processing necessary todetermine the wind, range, and ballistic offsets, as well providing thecentral control for the system. A battery with a power conditioner 700provides the required power to all sub systems. The function of allthese subsystems and components is described below in detail.

Prior to use, the laser beam may be aligned to the weapon barrel so thatwhen the weapon is aimed at a target, the laser spot will be on thetarget. Light reflected/scattered from this spot travels through theatmosphere and a portion of this light may be collected by the opticalreceiver, which is comprised of the wind measurement channels 500, 600.In an exemplary embodiment, the receiver has two equal size apertures102 and 103 mounted horizontally for measuring crosswind in onedimension (i.e., horizontal crosswind). Because of the atmosphericturbulence, the intensity of the light is modulated at the windmeasurement channels 500, 600 creating a pattern of randomly sizedbright and dark spots. This pattern is carried over by the pathcrosswind from one receiver to the other. The optical lens at aperture102 of wind measurement channel 500 focuses the time varying pattern onthe camera pixels or in a different configuration on a single PINphotodiode or APD (avalanche photodiode) detector. Similarly, the lensat aperture 103 of wind measurement channel 600 focuses the pattern onthe camera pixels, PIN photodiode or APD. Due to the motion of the pathcrosswinds, the pattern arrives at the lens of aperture 103 at aslightly different time than at lens of aperture 102. The separationbetween the two apertures 102, 103 divided by the calculated transittime of the signal is proportional to the magnitude of the pathintegrated crosswind. The direction of the crosswind is determined bythe transit direction of the pattern from aperture 102 to aperture 103.The design requires careful selection of the receiver separationallowing measurement of a large range of crosswinds while ensuring thescintillation pattern stays undisturbed during transit from aperture 102to 103. In the preferred embodiment, if the center to center separationis selected to be approximately 50 mm, it is possible to measurepath-integrated winds from 0.5 m/s to over 15 m/s.

To determine the transit time, a covariance technique may be used tocalculate the downrange path integrated crosswinds from the detectionresults of the two wind measurement channels 500, 600. Four potentialcovariance-based techniques, any of which can be used for this purpose,are described in Wang et al., “Wind measurements by the temporalcross-correlation of the optical scintillations,” Applied Optics V20,No. 23, December 1981, the contents of which are herein incorporated byreference in their entirety. All have their strengths and weaknesses. Inone embodiment, the peak shifting method may be employed, and augmentedwith various noise filtering and signal averaging methods to improve theSNR. Utilizing the peak method, the time to transit between apertures isdetermined by calculating the cross covariance function of the twosignals and noting the time at which the function peaks. The crosswindmay then be calculated as the effective aperture separation divided bythe time delay to the peak. A single wind measurement may typically bemade within 0.5 to 1 seconds. When the user pushes a switch 1000 toinitiate a wind measurement, the system first records several frames(nominally 10) with the laser off. The laser may then turn on, and thedata acquisition may continue until a set number of frames have beenrecorded. The laser-OFF frames are averaged together and subtractedpixel-by-pixel from each of the laser-ON frames in order to remove anybackground light from non-laser sources that does not change with thewind. After background subtraction, the processor 1300 may identify thelocation of the laser image spot by its signal level above the otherpixels. Because the laser spot is spread over several pixels the signalsfrom all of the pixels with laser illumination are summed to give thetotal power captured by the receiver's pupil 102, 103.

The aforementioned embodiment utilizing the peak method provides aweighted average of the crosswind along the path from the shooter totarget. Some portions of the path have a greater effect on the averagethan do others. The same is true of the crosswind effect on thedeflection of the projectile. Wind near the shooter affects thetrajectory more than the wind near the target. The effect is quantifiedin the ballistic influence function. Ideally, the wind measurementweighting function is designed to match the ballistic influencefunction. By optimizing the receiver aperture size and the divergence ofthe laser beam, an approximate match of the weighting function is madeto the ballistic influence function, as will be described in more detailbelow in connection with FIG. 15.

Once the path average integrated crosswind has been measured (by usingthe peak method or another cross-covariance technique), it may beprocessed according to a ballistic aim-point calculating software thatprovides an offset aim point (OAP). Various types of software, which arecapable of providing such an OAP using the measured path integratedcrosswind, are commercially available including PRODAS (by Arrow TechAssociates Inc.), ATrag™ (by Horns Vision, LLC) and others. When the OAPis determined, it is displayed by the micro-display 1100 to the users207 via the eye piece 105 for re-aiming the weapon. Before an OAP can becalculated, inputs from the local sensors 800 (such as a digitalmagnetic compass 801, global positioning system 807, and/or theatmospheric data sensors 802, 803, 804 illustrated in FIG. 13),ammunition data, and the range may also be required by the ballisticcalculator. In an exemplary embodiment, the process of calculating theOAP may be performed in near real-time using standard FPGAs(field-programmable gate arrays), and the result is updatedapproximately every 0.5 sec or faster. In addition, each update of theOAP may be accompanied by a confidence metric, which is determined asthe standard deviation of the wind measurements up to that update usinga cross-covariance technique. This confidence metric may also bedisplayed in the user's sight, e.g., in the form of various shapes suchas converging arrows, circle, etc. In the example where the confidencemetric is comprised of converging arrows, as measurements continue andthe SNR (signal to noise ratio) improves, the arrows may converge toapproximately the size of the target. The user may at any time choose tofire the weapon, or wait for a better condition to get the first roundhit.

Laser System

FIG. 3 shows the components of the laser system 300 consisting ofsuitable laser diode 123, collimation optics 107 and, in someimplementations, a laser beam steering assembly 106 to redirect the beamuseful for boresighting when the XWS is used along with an existingriflescope. The wavelength of laser selected may be limited to thevisible band to prevent the system from being used by terrorists incombat situations. Visible wavelength of the laser deters terrorists forusing such system in combat situations where it can be easily detected.In an exemplary embodiment, the wavelength of the laser diode 123selected is 638 nm which also allows use of a low cost CMOS image sensoras the signal detector(s) (113 in FIG. 5, 119 in FIG. 6) for the windmeasurement channels 500, 600. Other blue or green wavelengths can alsobe used if low cost laser diodes with average output power greater than170 milli-watts are available. The light source selected is asingle-mode pseudo coherent diode laser 123 which is low cost,efficient, readily available, and has a very short coherence length,approximately 400 microns. High coherence light, with coherence lengthgreater than a couple of centimeters, produces speckles at thecollection receiver when it reflects from a rough surface (>500 micronsrms). Speckles are produced due to light interference caused by thesurface roughness which minutely changes the path length of the variousreturning beams at the receiver. The laser speckle pattern looks similarto the spatially varying intensity pattern caused by atmosphericturbulence. The speckles however, do not move with the wind butintroduce noise in the time varying wind signal. Various methods may beused for reducing the speckle contrast and the resultant noise level,including, but not limited to the following. One technique is splittingthe laser beam among multiple beams whose path lengths to the targetdiffer by more than the coherence length of the laser. This results inspeckle patterns for the various beams that are uncorrelated. Additionof the uncorrelated speckle patterns yields a reduction of the specklecontrast. An alternate technique is to split the laser beam between twobeams with orthogonal polarizations. This results in two uncorrelatedspeckle patterns that when added together reduces the contrast. Anotherapproach employs wavelength chirp to widen the spectral width. Whenlaser diodes are pulsed, the wavelength of excitation frequency changesby more than 100 GHz over a 200 ns pulse-width. When semi-conductorlaser diode sources are frequency swept at these rates, the lasercoherence length is reduced (spectral line width is increased) whichreduces the speckle contrast. Whatever method is used to reduce thecoherence length, it is preferred that the coherence length be less thana surface roughness associated with the target. For example if thespectral line width is 4 to 8 nm, the coherence length is=(6.38e-7)²/8e-9=51 μm which is generally less than the rms (root meansquare) of surface roughness of a target (typically on the order of 500microns).

The laser diode 123 has a wavelength of 638 nm and typically a 28 mmdiameter lens accepts the diverging radiation from the diode 123 whichis then collimated to a 50 micro radian beam. To maximize lightcollection and to produce diffraction limited beam divergence, carefulselection of the lens focal length provides optimized performance Oncecollimated according to the optics in laser collimator 107, the laserbeam is propagated through a beam steering module 106 to align it withthe imaging optics, weapon bore or riflescope crosshair. In an exemplaryembodiment, the laser system 300 operates in three modes: low lightlevel imaging, ranging and wind measurement. All the modes areaccessible to the user via switches on the system housing 1000.

A beam steering module 106 may be used in the collimated laser path toallow precision alignment of the laser beam in two orthogonal directionswith the imaging optics of an existing riflescope mounted to the weapon.In an exemplary embodiment, the steering module 106 is a set of Risleyprisms which provides better than 50 micro radian angular alignmentresolution and stability from shock, vibration and temperature changesin retaining the alignment of the laser beam over long periods. Therobustness of this approach stems from the fact that the Risley prismsprovide a mechanical compression in the angular steering of the laserbeam. For example, if two counter-rotating Risley prisms aremechanically rotated by 180 degrees, the laser beam may be movedlinearly, only a degree, in one direction providing a compression of 180times. To make the laser alignment user friendly, the design requiresmoving the laser beam linearly in the azimuth or elevation directionindependently so that the desired position of the beam in angular spaceis achieved rapidly. To achieve this functionality, two sets of counterrotating Risley prisms are mechanically configured so that they may berotated by two independent set of knobs (illustrated by referencenumbers 108, 109 in FIG. 8). In an alternate embodiment, a piezoelectricdriver can also be used to move the laser diode in azimuth or elevationrelative to the collimating lens producing similar beam alignment. Inanother embodiment, a micrometer mechanical stage could be used to movethe collimated laser beam assembly in the azimuth and elevation. Themajor disadvantage of both these approaches is that they are extremelysensitive to shock and vibrations causing large changes in the beamalignment over time.

In an exemplary embodiment, an electronic crosshair may be generated inthe XWS 100 for aiming the system on the target. Looking at a targetusing the crosshair in the view finder, the angular misalignment offsetbetween the rifle bore and the system's laser aim point (i.e., thesystem line of sight (LOS)) may be determined. Because the ballisticoffset aim point calculated using the wind and range measurements isindependent of the system LOS, factory calibration of the XWS and riflezeroing can yield the required offset without moving the laser beamusing a steering device. This will be described in more detail inconnection with weapon and system alignments below.

Optical Receivers

According to an exemplary embodiment, the XWS 100 includes three opticalreceiver channels 400, 500, and 600. The components of each of thesechannels 400, 500, and 600 are shown in FIG. 4, FIG. 5, and FIG. 6,respectively. As discussed above, two of these optical channels 500 and600 are wind measurement channels configured for receiving the reflectedsignal from the target to measure wind (and receiving image signals ofthe area surrounding the target for imaging). As such, the third channel400 may be used for ranging measurements.

Ranging:

In an exemplary embodiment, a separate aperture 104 is used with a lensthat focuses the received narrow ranging pulses with a width of up to 70ns onto 3 to 10 MHz bandwidth silicon APD (avalanche photodiode)receiver 122. In alternate embodiment, one of the wind measurementapertures 102, 103 could be used for ranging by inserting a beamsplitter but this has the penalty of losing a part of the wind signal.The light passes thru a narrow band-pass filter to exclude backgroundsunlight minimizing the shot noise generated while maximizing the signalto noise ratio. Range is determined by conventional round trip time offlight measurement known to those familiar in the art. Range accuracy isimproved by both averaging multiple single pulses in flight to improveSNR and by interpolating the received signal pulse using for example a200 MHz clock to yield a total range uncertainty of ˜1.5 meters.

During ranging, the laser 123 is rapidly pulsed at a high peak power forperiods up to a second. The returns from the outgoing pulses are timedto give the range as is practiced commonly in laser rangefinders. Toimprove the SNR the returns from multiple pulses are averaged. Theaveraging time varies with range and conditions. The ranging process isautomatically terminated once a result with an SNR sufficient to assurerequired accuracy has been achieved. For example, the laser can bepulsed with a peak power of 300 mw, pulse width 70 nsec, and 125 kHzrepetition rate. Using multiple-pulse averaging the link budget closesout to 1000 meter with an averaging time of approximately 500milliseconds. The time reduces to less than 1 millisecond for a 200meter range. Once the range is determined with the desired accuracy (seeoperation 3202 of FIG. 12, which will be discussed in more detail below)the range can be displayed to the user (e.g., as part of indicator 5003in FIG. 19), and the laser is automatically shifted to the windmeasurement mode (see operation 3300 of FIG. 11, which will be discussedin more detail below). In an alternate embodiment, the range measured byother techniques can be manually input into the system.

Wind Measurement Channels:

FIG. 5 and FIG. 6 show the components of the two wind measurementchannels. In the wind measurement mode the laser output is a constantcontinuous power and the laser return from the target is detected by thewind receiver channels. Two 35 mm lenses 102, 103 separated by a fixedhorizontal distance (e.g., ˜50 mm) gather the light for wind measurementand focus it onto a corresponding pair of CMOS image sensors 113, 119.The image sensors can be read out at rates up to and greater than 1000frames/sec to insure capture of the frequency content of the atmosphericscintillation dynamics. The signals used for the wind measurementcovariance calculations are derived by summing the outputs of all thepixels in a small area surrounding each laser spot 116. An alternativeembodiment employs a series of mirrors to direct the images of the laserspot from both apertures onto different areas of a single CMOS imagesensor. In another alternative embodiment the CMOS image sensors arereplaced by silicon APDs or PIN photodiodes. For each of theaforementioned embodiments, the background sunlight may be filtered outby band-pass filters 112, 118 (e.g., 10 nm bandwidth) to accommodateacceptance angle and change in wavelength of the source overtemperature. Cold mirrors 111, 117 are used in the receiver optical pathto pass the laser wavelength to the image sensors but reflect the restof the visible spectrum onto a color 120 or black and white digitalcamera for target imaging.

As described earlier, any of the four different covariance processingtechniques described in Wang et al., (“Wind measurements by the temporalcross-correlation of the optical scintillations,” Applied Optics V20,No. 23, December 1981, the entire contents of which are hereinincorporated by reference), may be used to provide a path-weightedaverage value of the wind. In each of these techniques, the weightingfunction is dependent on: the laser beam divergence and separationbetween the two receive apertures, the wavelength of the laser light andthe range. When the size of the laser spot (beam divergence) is muchgreater than the lens separation the weighting function peaks close tothe shooters end of the path and has little sensitivity at the far end.If the size of the laser spot is significantly smaller than the lensseparation, the sensitivity of the weighting function peaks near thetarget end. Proper choice of the system parameters provides a weightingfunction for the wind measurement that approximately matches theballistic influence function of the bullet.

In some situations, when the winds are uniform along the path, theaverage of a single weighting function is adequate to provide accurateresults. In other situations, when the winds are not uniform due tonon-uniform terrain, buildings, trees, or hills that block or channelthe wind, the accuracy of the single weighting function approach issomewhat degraded. Accuracy in the non-uniform wind case can be improvedby making measurements with multiple different weighting functions, andcombining the results.

The different weighting functions can be generated by changing theseparation between the two receiver lenses 102, 103, or by changing thelaser divergence to change the spot size of the laser, or else byanalyzing the angular profile of the laser spot size. Changing theseparation of the receivers during wind measurement of about 1 sec isconsidered impractical. However, the other two approaches are practicalwith each having their own limitations. The change in laser beamdivergence can be achieved by using multiple fixed divergence beams(which requires additional light sources, thus impacting costs) or witha piezo driven positioner on the collimator axis of a single lasersource. FIG. 15 shows the contribution to wind measurement from fourdifferent weighting functions which are produced by varying the laserdivergence from 50 to 500 μrad. Although FIG. 15 shows four differentweighting functions, it will be readily understood that by increasingthe number of weighting functions, the crosswind profile resolution canbe improved.

In another embodiment, instead of varying the laser beam divergence, itmay be easier to analyze the angular profile of the spot size in theimage plane of the camera using software instead of adding hardware. Thelaser spot is imaged onto the image sensor of the camera and spansseveral pixels. The outputs of all the illuminated pixels are summed toget the signal for chosen beam divergence. As shown in FIG. 16, thesignals corresponding to different beam divergences can be derived fromthe single image data by sequentially summing the pixels in differentareas of the image. For example, the areas 6001, 6002, 6003, 6004 inFIG. 16 could all be processed as different divergences. Thus, multiple“effective” spot sizes can be extracted from data collectedsimultaneously. Summing all pixels within the largest circle 6001corresponds to a spot size of 100 μrad (micro-radians), all within thenext smaller circle 6002 correspond to spot size 80 gad, next smaller6003 is 40 gad and smallest 6004 is 20 grad.

Target Imaging:

In the embodiment illustrated in FIG. 5 and FIG. 6, in one windmeasurement channel 600 the cold mirror 117 reflects the light onto ahigh resolution image sensor 120, and in the other wind measurementchannel 500 the light is directed to an absorbing surface 114. In anexemplary stand-alone embodiment, the XWS 100 is used without riflescopethus eliminating the associated cost. The color camera 120 runs atnormal video rates (15 to 60 Hz) and provides the user a view of thetarget and the surrounding area comparable to that seen with aconventional riflescope. The output image is viewed on a high resolutionmicro-display 1100. The micro-display 1100 is viewed through an eyepieceglass 105 located where the conventional riflescope eyepiece wouldotherwise be, providing an eye relief ranging from 2 to 4 inchesadequate to prevent injury to the user 207 eye from gun recoil. FIG. 8shows an external view of an embodiment of the XWS 100 which can bemounted directly to the rifle.

The image resolution that can be provided by the “digital scope” iscomparable to that of existing riflescopes because both are limited bythe diffraction from the aperture size. Assuming a 35 mm riflescope and35 mm apertures for the XWS both will have a 38 μrad resolution limit at550 nm wavelength. Assuming for an example that the camera sensor 120 isan Aptina™ AR1411HS with a 4620×3084 pixel array and a 2.86 μm pixelpitch, and the receiver lens is 35 mm diameter with a 100 mm focallength; the instantaneous field of view (IFOV) of a single pixel will be28.6 μrad. This is less than the 38 μrad diffraction-limited resolution.The system will therefore be aperture diffraction limited rather thanpixel limited providing a smooth image without pixel grains.

If the entire pixel array is displayed to the user 207, the FOV (fieldof view) would be 7.6×5.1 degrees. If instead, the image displayed tothe user is circular to emulate the view normally seen through ariflescope, the FOV may be 5.1 degrees in diameter. This FOV can be usedfor searching a target. For closer inspection and aiming at the target,a pushbutton switch can be activated to apply digital zoom in a seriesof steps.

Nighttime imaging without the use of an image intensifier night-visiondevice is made possible by a clip-on holographic diffuser, which may bedisposed in the path of collimated laser beam 130 (FIG. 2), to providedual divergence of 44 milli-radians for illumination while retaining theoriginal central divergence of 50 μrad. The illumination beam is veryuseful in finding the target, whereas the narrow spot beam verifies thatthe system is aligned and that the beam is on the target when ameasurement is initiated. In this mode, the laser energy per frameincreases due to lower camera frame rate, and the camera is set to binpixels. Link budget calculations show that even with no moonlight,images with SNR>10 dB can be achieved out to a range of 200 meters. Toachieve this, the following parameters may be required: laser power of170 mw, beam divergence of 2.5 degrees, 4×4 pixel binning, and a cameraframe rate of 15 frames/sec. The 2.5 degrees beam divergence becomes theFOV of the useable image, 8.7 meters in diameter at a range of 200 m.

Electro-Optic Design

In summary, the electro-optic system, which is configured in anexemplary embodiment of the invention to measure path-integratedcrosswinds, uses at least two apertures 102 and 103 to collect the laserlight 116 scattered back from the target 115 onto the digital winddetectors 113, 119. In an exemplary embodiment, the digital winddetector 113, 119 is a CMOS camera. Camera pixels illuminated by thelaser spot 116 are summed into a single measurement (see operation 3307of FIG. 12) that changes with time and moves from one aperture to theother with the crosswind. By measuring the covariance of the tworeceived signals, the path-averaged crosswind is calculated using thecross-covariance processing technique. The two aperture approach is morerobust than the single aperture approach because it does not requireresolving the light and dark features on the target, and is unaffectedby the atmospheric turbulence noise on the outbound path and isinsensitive to scintillation index changes since the entire laser spotis summed into a single signal.

In an exemplary embodiment, the electro-optic system is designed tomeasure range and crosswind up to 1000 meters. This requires a laser 123with CW power of 170 milli-watt at 638 nm laser wavelength with beamdivergence=50 μrad produced by a 28 mm collimating lens and thereflected light collected by a 35 mm receiver lens (same as a commonlyused in deer hunting riflescope) with a camera frame rate up to 1000 Hzand noise floor 25 e/pixel/frame. This design is optimized by conductinground trip optical power budget analysis for required signal noise ratioof 10 dB while also optimizing the weighting functions along the path toclosely match the influence function of the projectile. Other laserwavelengths in the visible or near invisible (Infrared/IR) range couldalso be used but the design would be somewhat different. For ranging,the same laser provides a peak power of 300 milli-watt at 70 ns pulsesthat may be repeated at 125 kHz to ensure a single pulse in flight tothe target and back. The returned signal is collected with a 15 mmaperture lens that focuses the signal on a 0.5 mm APD receiver with abandwidth of 3 to 10 MHz. The system parameters for both ranging andwind measurement modes in the preferred embodiment, are shown below inTable 1.

TABLE 1 System Parameters to Close Link Budget at 1000 m Range WindParameter (units) Ranging Measurement Range to Target (m) 1000 1000Laser Peak Power (w) 0.3 0.17 Wavelength (nm) 638 638 Beam Divergence(μrad) 50 50 Waist Diameter (m) 0.028 0.028 Pulse Duration (sec)   7 ×10⁻⁸ CW Pulse Repetition Rate (kHz) 125 NA Receiver Aperture (mm) 15 35Camera Frame Rate (Hz) NA 750 Duty cycle 8.75 × 10⁻³ 1 Energy/Pulse (J) 2.1 × 10⁻⁸ NA Signal Averaging Time (sec) 0.30 0.002 Energy/Frame (J)NA 4.0 × 10⁻⁴ Receiver Noise Floor (dBm) @ 3 MHz −77.9 NA CMOS Cameranoise floor (e) NA 25

Digital Riflescope Design

The objective in an exemplary embodiment of the invention is to replacethe conventional riflescope with a system that provides automaticcompensation for wind and range errors while providing opticalperformance that matches or exceeds that of conventional riflescopes.Accordingly, in an exemplary embodiment of the present invention, theXWS 100 can be implemented as a “digital riflescope” or “digital scope,”providing the user with a micro-display 1100 for viewing the target 115and surrounding area (e.g., with the determined OAP and other calculatedparameters overlaid). FIG. 7 is a block diagram illustrating thecomponents of the XWS 100 arranged as a digital scope 200.

Diffraction sets an ultimate limit to optical performance ofconventional riflescopes. Diffraction limited resolution is inverselyproportional to aperture size so resolution improves with the size ofthe aperture. In view of this, we compare the performance of a XWS-based“digital riflescope” 200, designed in accordance with principles of thepresent invention, to a conventional riflescope with the same aperturesize. A series of products with varying performance can be fielded tocompete with the range of conventional scopes currently available. Forour comparison, we assume an aperture size of 35 mm. The diffractionlimited resolution for a 35 mm aperture is 38 μrad which becomes theeffective resolution of both the riflescope and the XWS-based digitalriflescope 200. Other performance parameters are FOV and magnification.For a typical 35 mm riflescope the magnification zooms from 2.6× to 7.8×with corresponding FOV from 7.2 to 2.6 deg.

Components of the digital scope 200 shown in FIG. 7 will now bedescribed. An objective lens 202 focuses the image onto an image sensor203. The output of the image sensor is read by a signal processor 1300(as described above in connection with FIG. 2) and electronicallydisplayed on a micro-display 1100 (also described above in connectionwith FIG. 2), which is viewed by the user through a magnifying eyepiece105 (as shown in FIG. 2). According to an exemplary embodiment, theobjective lens 202 and image sensor 203 may be implemented as part of animage-capturing channel separate in addition to the wind measurementchannels 500 and 600 (not shown in FIG. 7). Alternatively, it ispossible for these elements to be shared with one of the windmeasurement channels, such as channel 600. For instance, the objectivelens 202 of FIG. 7 may refer to the same objective lens 103 utilized bythe wind measurement channel 600, as illustrated in FIG. 2 and FIG. 6.Also the image sensor 203 of FIG. 7 may correspond to the same imagesensor 120 used in the wind measurement channel 600, as illustrated inFIG. 2 and FIG. 6.

Table 2, provided below, shows the calculated performance parameters ofa XWS-based digital riflescope 200 designed to replace a 35 mmriflescope. The key parameters to achieve images indistinguishable fromthose seen through a conventional riflescope are number of pixels andthe pixel pitch of both the image sensor and the micro-display. Theparameters used in Table 2 list the state of the art image sensor andmicro-display. For both components the performance will improve withtime while the cost declines.

TABLE 2 Objective Lens Diameter D (m) 0.035 Focal Length F (m) 0.1 f#2.8 Diffraction Limit (μrad) 38 Image Sensor # Pixels 4620 × 3084Aptina ™ AR1411HS Pixel Pitch x (μm) 2.86 IFOV (μrad) 28.6 Total FOV(rad) 0.13H × 0.088V Total FOV (deg.) 7.6 × 5.1V Micro-display # colorpixels 1920 × 1200 eMagin ® EMA-100820 Pixel pitch (μm) 9.6 DisplayWidth (mm) 18.4 Eyepiece Magnification 5X Eye Relief (mm) 100 SystemCircular FOV At Mag = 3.7x (deg.) 5.1 At Mag = 18.5x (deg.) 1.0

The display assembly consists of an eyepiece 105 through which thehunter views a high resolution micro-display 1100. The micro-display1100 is driven by the processor 1300 and provides a digital image of thetarget 115 and surrounding area. During wind measurement, the laser spot116 is also shown. FIG. 19 illustrates the view of the target area asseen through the micro-display 1100 according to an exemplary embodimentof the present invention. As shown in this figure, the image of thetarget 115 and laser spot 116 may be overlaid with a reticle 5001(crosshair) and an offset aim point 5007 showing the correctionsrequired to compensate for wind and range. The overlay information mayinclude any combination of the offset aim point 5007, confidence metric5004, 5005, a collection of reference marks 5009 that assist angularmeasurement relative to the offset aim point 5007, and moving targetlead marks 5008, that assist in aiming at moving target. The image thatis displayed may be produced by the wind measurement channel 600 of FIG.6. An alternative to the eyepiece viewing, it would be possible to mounta larger direct-view digital screen, such as used in camera-phones.However, the eyepiece display has the advantage of closely mimicking theway users have learned to shoot with a riflescope. Other informationsuch as the range 5003 to the target can also be displayed. Themagnification of the image can be selected by the user through a singlepushbutton that steps the digital electronic zoom thru a series ofvalues.

Controller Processor

The signal processor 1300 is the central control for the XWS 100. It mayreceive commands from the user via a series of pushbutton switches 1000.It may provide the switching between imaging and wind measurement, thedigital zoom, control of the laser current required for either ranging,illumination, or wind measurement, collects and stores information onall parameters needed for the ballistic calculations, acquires theimage, wind, and range measurement data, and met data, analyzes the datato determine range and crosswind, displays the image and range, and theaim point offsets.

In an exemplary embodiment, when the system 100 is operated without theriflescope, the digital camera provides imaging at video rates for usein target acquisition. FIG. 11 is a flowchart illustrating an algorithmfor performing a sequence of measurements 3000, and displaying theresults thereof, according to an exemplary embodiment. Once a target islocated, the user can press a button 1000 to initiate a measurementsequence (2300). After some initial data collection steps (2200 and3100), a decision is made as to whether a ranging measurement orcrosswind measurement is selected in 3001. If a range mode measurementis selected in 3001, then the XWS 100 measures the range in 3200 anddisplays the range in 3201. If, on the other hand, the wind mode isselected in 3001, then the system 100 automatically measures the rangeto the target in 3200, and then shifts into the wind measurement modewhich commences in 3300. The image sensors 113, 119 used for windmeasurement are operated in a high frame rate windowed mode where only asmall area of pixels surrounding the laser spot is read-out. The imagesensors 113, 119 are synchronized so that there is no time delay errorintroduced into each of the two output signals.

Wind Algorithms:

FIG. 12 is a flowchart of the algorithm for obtaining one or moremeasurements, but focusing on particular operations for obtaining asingle measurement of the crosswind, according to an exemplaryembodiment. As discussed above, the optical receiver for windmeasurement consists of two channels 500 and 600, each having acollection lens 102, 103 and connected to respective image sensordetectors 113, 119 (or possibly the same image sensor detector). In eachof these wind measurement channels 500 and 600, the lens 102, 103focuses the laser spot onto a small area of the image sensor. A singlewind measurement involves capturing the time history of each channelssignal for a fixed time, tw. As described above signal processingmethods based on a cross-covariance technique have been demonstratedcapable of extracting the crosswind speed from the temporal behavior ofthe two signals detected by the respective wind measurement channels 500and 600. Wang et al., (“Wind measurements by the temporalcross-correlation of the optical scintillations,” Applied Optics V20,No. 23, December 1981, the entire contents of which are hereinincorporated by reference) compared the strengths and weaknesses ofseveral different approaches. The peak shifting approach as described inWang et al. has been found to be advantageous under variouscircumstances, but any of the other techniques described in Wang et al.could also be implemented in such manner as to provide similar results.

FIG. 17 illustrates examples of respective signals 3308, 3309 which aredetected, at different times, via the first and second wind measurementchannels. In this figure, the two signals 3308 and 3309 are shown asexactly the same except for the time difference or delay. However, inpractice, the correlation between the two signals gradually decays withtime. This decay is the source of errors that result in differences inaccuracy of the different signal processing approaches. Regardless ofwhich cross-covariance processing approach is employed, the wind isdetermined from the time varying signals that are proportional to thetotal signal collected by the respective apertures 102, 103.

Referring again to FIG. 12, a description of the algorithm describedtherein will now be provided. It is noted that, in FIG. 12, for purposesof convenience, the wind measurement channel 500 is referred to aschannel A, while wind measurement channel 600 is referred to as channelB (this is consistent with FIG. 2 which refers to channel 500 ascorresponding to “Imager A” and channel 600 as corresponding to “ImagerB”). The measurement sequence of FIG. 12 begins at 2300 where the userrequests a measurement of either range or wind (e.g., via push button1000). If the request is to measure range, processing may proceed to thesequence of 3203, 3200, and 3202 in order to measure and display therange. If instead the request is for wind measurement in 3001 of FIG.12, the processor 1300 proceeds to measure crosswind. However, even if3001 decides to perform the crosswind measurement, the first action maystill automatically be to measure the range in 3200 and then display themeasured range in 3201, as illustrated in FIG. 11. In 3302, the imagesensors for both wind measurement channels acquire a small number offrames (nominally 10) with the laser off. The laser is then turned on in3304 in the CW (continuous wave) mode to acquire data for the windmeasurement. With the laser on, both channels 500(A) and 600(B) acquirea large number m (≈300) of frames in 3305. The n background frames arethen averaged, pixel by pixel, to yield an average background frame foreach channel in 3303. Accordingly, in 3306, the average background frameobtained in 3303 for wind measurement channel 500(A) is subtracted fromeach of the m frames acquired by the same channel 500(A) with the laseron. Similarly, in 3306, the average background frame obtained in 3303for wind measurement channel 600(B) is likewise subtracted from theframes detected by channel 600(B) in 3305 (i.e., with the laser on).Thereafter, the processor 1300 identifies the location of the laser spotimage for each of channels 500(A) and 600(B), and sums the signal fromall of the pixels that are illuminated by the laser spot for each ofthese channels in 3307. This yields the two time-series signals S_(i)(A)and S_(i)(B) in 3308 and 3309, respectively, where i indicates the framenumber which is proportional to time. The cross-covariance function ofthese time-series signals S_(i)(A) and S_(i)(B) is calculated in 3310,and an auto-covariance function of these signals S_(i)(A) and S_(i)(B)is calculated in 3311. At this point, FIG. 12 illustrates that there arefour possible techniques (each corresponding to one of 3312, 3313, 3314,and 3315) that can be selected for determining the crosswind, eachcorresponding to a particular cross-covariance technique as described inthe Wang et al. article. However, it should be noted that the inclusionof four different techniques in FIG. 12 (as indicated by thebranching-off of 3312, 3313, 3314, and 3315) is not intended to conveythat the processor 1300 is capable of performing four differenttechniques. Instead, techniques 3312, 3313, 3314, and 3315 in FIG. 12are merely intended to show that the principles of the present inventioncan be implemented using any of the four techniques, or any othertechnique that is similarly capable of processing the detection resultsof channels.

One particular embodiment may use the peak shifting method of 3314 and3318. This approach determine a time delay tp at which the covariancefunction has its peak value in 3314, and then determines the crosswindspeed W in 3318. This is done by dividing the value of the horizontalseparation ρ between the two channels 500(A) and 600(B) by the peakdelay time tp.

As an alternative to the peak shifting technique, a slope method may beemployed. If this technique is employed, the value of the covariancefunction slope at a time delay of zero is determined in 3315, and usedin 3319 to calculate the crosswind speed W from the expression W=kmultiplied by the slope at zero, where k is an instrument constant.

Another alternative for covariance processing is the Briggs methodoption, which determines the time t_(c) at which the cross-covariancecurve intersects with the auto-covariance function according to 3313.The crosswind speed W is then determined in 3317 by dividing thehorizontal separation p of the channels 500(A) and 600(B) by 2 times thecrossover time t_(c).

As shown in FIG. 12, another possible option for cross-covarianceprocessing is the Frequency method which, measures the width t_(f) ofthe auto-covariance function at the half power point in 3312, and usesthis value to calculate the wind speed W in 3316 according to theexpression W=k′/t_(f), where k′ is an instrument constant.

The covariance calculations in 3310 and 3311 may be carried out usingthe data collected over the time t_(w). To increase the measurementaccuracy, however, multiple measurements may be made and averaged (whilediscarding improbable wind thresholds) to give the final answer. Thedata acquisition may continue for as long as the user maintains pressureon the wind measurement switch. Calculation of the first windmeasurement result begins at t_(w) after the wind measurement switch isactivated, and the result is displayed once the data is processed. Thetypical value of t_(w) is 500 msec but depending on the design can belonger or shorter. During the processing the data collection maycontinue and, after the processing of the first measurement is complete,the second calculation begins. The data for the second calculationconsists of shifting the 500 msec selected for processing 50 msec aheadas illustrated in FIG. 18. With this approach, a 5 second data streamcan yield 90 measurements to be averaged to improve the accuracy of theresult. Depending on the conditions, the accuracy may be sufficientwithin the first second. Therefore, in addition to the wind result thedisplay also shows a confidence metric (e.g., with marks 5004 and 5005as shown in FIG. 19) based on the accuracy of the result as defined bythe variance of the wind results. When the variance is below a selectedthreshold, regardless of the averaging time, the user can accept thecurrent result with confidence that the probability of a hit will behigh. The user can then terminate the data acquisition and use thecurrent result.

It is contemplated that the crosswind can be measured with accuracybetter than 0.5 m/sec to insure a high hit probability with the firstround. Under most circumstances a point or local measurement of the windis seen to vary significantly faster than a second or so compared to theprojectile flight time of 2 to 3 sec. This would imply that the windmeasurement would be latent and inaccurate to get a hit. Fortunately,what determines the projectile deflection is the path-averaged windwhich changes at a much longer time constant than a second. The pathaveraged wind changes at a slower rate than the local wind because it isthe average of all sequential local wind fluctuations over the entirepath that the bullet experiences. For example if 1 m/s wind changesdownrange at different rates, the overall rate will be slower than theslowest rate.

The XWS 100 makes multiple wind measurements and computes a cumulativeaverage in order to reduce the measurement error which increases theaccuracy. The averaging improves the accuracy so long as the variance ofthe individual readings is dominated by measurement error. At some timedepending on the conditions however, the variance is due to actualchanges in the path averaged wind. Extending the averaging time beyondthis time increases the difference between the measured and the actualwind the projectile experiences in flight resulting in decrease of thehit probability.

The path-integrated wind, which is averaged over the time of projectileflight, changes with time. Field observations reveal that under someconditions the measured value remains effectively valid for 10 secondsor even longer. However, under other head or tail wind conditions, whenthe crosswinds are not full value, the value changes faster than 10seconds. To avoid “obsolete” measurement the averaging of themeasurements is done as a cumulative average up to 5 seconds and afterthat over the most recent 5 seconds of data. According to exemplaryembodiments of the invention, the measurements, calculations, anddisplay of the results may be automatically provided to the user withinseconds, thereby providing a distinct advantage over systems where thetime lag between wind measurement and firing time exceeds the timestability of the wind.

The signals derived by the summation of pixels surrounding the laserspots (e.g., in 3307 of FIG. 12) may include light from those spots dueto sunlight illumination as well as the laser illumination. In someconditions, the sunlight background exceeds the laser contribution. Toremove the common mode (sunlight) component from the signal prior tomaking the covariance calculation, the data acquisition for the windmeasurement mode begins by acquiring several frames with the laser notyet turned on in 3303 of FIG. 12. These frames are averaged andsubtracted (pixel by pixel) from the subsequent laser-on frames toremove the background component in 3306.

In an imaging mode, the sunlight background is the primary signal, alongwith the laser spot 116 on the target that provides confidence to thehunter that the laser 123 is aligned and has not shifted. To deal withthe fact that laser signal is small compared to the sunlight, the imagedisplayed to the user is the result of adding to the raw image data anenhanced laser image. The enhanced laser image is zero at all pixelsoutside of the area immediately surrounding the laser spot and for thosepixels within the spot the pixel values are set close to saturationvalues.

2D Wind Algorithms:

Exemplary embodiments of the invention, as described above, are intendedto measure the crosswind in the horizontal plane. In applications wherethere may be significant vertical winds, as well as horizontal winds,the aforementioned embodiments can be modified. Particularly, a thirdwind measurement channel can be added to the XWS 100 in such manner thatthe added channel is displaced vertically from the others 500 and 600.Thereafter, the same covariance techniques as described above inconnection with FIG. 12 could be modified to account for timedifferences pertaining to detection results of vertically-separatedchannels.

It is also possible to measure crosswind speeds in both vertical andhorizontal directions with a single aperture and channel. This approachtracks the movement of the scintillation pattern falling on the targetdue to the turbulence encountered on the trip from the shooter to thetarget. As described earlier, this technique requires a significantlymore powerful laser and suffers from noise created by the turbulenceencountered on the return trip. In the single-aperture approach, thecovariance calculations are made between the signals from individualpixel-pairs in both directions.

The present invention provides a method and an instrument to measure twodimensional (azimuth and elevation) downrange winds from the shooter tothe target, by integrating and averaging the effects of the wind changesand direction similar to that experienced by the bullet. It is notedthat the winds in a third dimension, i.e., head or tail winds, may beexperienced. However, such winds have a small effect on the hit pointunless the shooter is moving rapidly at speeds greater than 30 metersper second toward or away from the target

Tracking Moving Targets:

For moving targets, the XWS 100 may also be configured to track thetarget and generate a lead offset to compensate for the movement that isdisplayed as an icon 5008 (FIG. 19) to the user 207. The laser beam spot116 as seen on the target image 210 would be used to track the targetsposition by moving the rifle to keep the laser spot 116 on the targetimage 210 (FIG. 7). The tracking angles could be measured by a gyroscope806 (FIG. 13) that would provide the angular motion as a function oftime allowing calculation of the required compensation offset anddisplayed to the user 207 by moving the OAP (offset aim point) 5007(FIG. 19) or as a lead icon 5008 (FIG. 19)

Ballistic Calculator

Now reference will be made again to FIG. 11. Particularly, after thetarget range has been measured according to 3200, and/or a path-weightedaverage crosswind has been measured according to 3300, various ballisticcalculations are performed in 3400, e.g., to determine an offset aimpoint 5007 to be displayed, e.g., on the target image.

According to an exemplary embodiment, the processor 1300 may perform anynecessary ballistic calculations by solving well-known equations ofmotion for a projectile in flight. Such equations, and standard solutionmethods, can be found in McCoy, R., L., Modern Exterior Ballistics,Schiffer Military History, Atglen, Pa., 2012, the entire contents ofwhich are herein incorporated by reference. The inputs to theseequations may come from a combination of the target range and crosswindmeasurements, as described above in connection with FIG. 11 and FIG. 12,as well as information collected by the XWS 100, e.g., according to theprocess illustrated in FIG. 10.

FIG. 10 is a flowchart illustrating a process 2000 whereby the XWS 100collects data which, in combination with the measured target range andcrosswind measurements, can be used to perform ballistic calculationsaccording to 3400 of FIG. 11. As shown in FIG. 10, after the XWS 100 ispowered on in 2100, a data initialization process may be performed in2200 to collect initial data. After data initialization, the XWS 100 mayoperation in a standby mode as shown in 2007. In this standby mode, adata setup operation may be initiated, automatically or by useroperation. Particularly, various types of setup operations may beperformed including those collecting data useful for ballisticcalculations. Such setup operations may include a Weapon Setup operation(2400) which receives input data on the relevant weapon, such as barreltwist rate and height of the center of the scope above the center of thebore; and a Bullet Setup operation (2500) which receives input data suchas muzzle velocity, bullet mass, bullet diameter, bullet length, custombullet drag coefficient vs. Mach number curve, and bullet ballisticcoefficient to calculate drag coefficient vs. Mach number using standarddrag curves.

In FIG. 10, the user may also perform various setup operations toindicate various preferences, e.g., regarding the display setup (2600),reticle setup (2700). Other types of possible setup operations may berelated to system and weapon alignment, such as the Weapon Zero (2800)and the Laser Zero (2900) operations which will be described in moredetail below. In FIG. 10, after the setup data is collected, the XWS 100may again enter the standby mode of 2007. In standby mode, a decisionmay also be made to start operating the image display functionality ofthe XWS 100. Accordingly, the image sensor may be turned on in 2002, theappropriate reticles may be displayed overlaid on the target image in2003, and the XWS 100 may be set in a ready mode in 2008 to wait for auser command to perform target range and crosswind measurements in 2300.However, in the ready mode (2008) of FIG. 10, the system 100 may revertback to standby mode (2007) in case of inactivity for a certain amountof time.

Now, referring again to ballistic calculations of 3400 in FIG. 11, othertypes of data in addition to the weapon setup data and the bullet setupdata may be needed. These additional types of data may be collected fromthe local sensors 800 (FIG. 2). FIG. 13 illustrates particular types oflocal sensors 800 which may be used by a processor 1300 to perform theballistic calculations. For instance, the processor 1300 may be able toobtain weapon heading data from the digital magnetic compass 801, firinglatitude from the global positioning system 807, weapon elevation angleand weapon cant angle from the accelerometers 805; atmospheric pressuresensor 802, atmospheric humidity sensor 803, atmospheric temperaturesensor 804, and a rate of change in weapon heading from theaccelerometers 805.

Using as inputs the aforementioned collected data, in combination withthe target range and path-weighted average crosswind measured accordingto FIG. 12, the processor 1300 performs the ballistic calculations of3400 of FIG. 11. Based on these calculations, the processor 1300 may beable to calculate the angular difference between a bullet zero and theaim point that would cause the projectile to hit the target at thedesired location in 3401. Accordingly, these angular differences aredisplayed (5003 in FIG. 19) as the holds in 3401, and used to calculatethe location of the offset aim point in 3402. Also, if the user 207 istracking a moving target with the weapon, accelerometer data (805 inFIG. 13) may be used to update the calculated holds 3401 and the offsetaim point 3402 with the calculated lead angle 5008.

Switches

The user may interface with the XWS 100 using pushbutton switches 1000(FIG. 2) that allow selection of ON/OFF, initiate the range measurementin 3200 (FIG. 11) and the crosswind measurement in 3300 (FIG. 11), alignlaser zero in 2900 (FIG. 10), align bullet zero in 2800 (FIG. 10), setreticle preferences in 2700 (FIG. 10), set display preferences in 2600(FIG. 10), enter bullet setup in 2500 (FIG. 10), enter weapon setup in2400 (FIG. 10), set magnification, and illuminate target.

Alternatively, all of the setup and calibration data could be receivedvia a wireless I/O system 900 (FIG. 2), e.g., from a wireless handheldcomputing device. Particularly, FIG. 14 illustrates various componentsin the wireless I/O system 900, which can be used to receive input foroperating the system apart from the buttons 1000 mounted on the XWS 100,e.g., via a wireless control unit 906 or a computing device 905.

Battery and Power Conditioner

The system 100 runs on battery power 700 for extended periods. Becauseit is implemented as a portable rifle mounted system in exemplaryembodiments, the weight of the battery is a significant parameter.Hence, minimizing the power draw is a prime objective. The internalscope design with the camera image sensor 120 as detector allowsoperation with a 170 mw CW laser. The laser 123 does not requirecooling. This significantly increases the electrical efficiency togreater than 25% and hence reduces power draw from the battery. Lowpower capability is made possible by the use of multiple apertureapproach rather than single aperture approach and using a CMOS imagesensor 113, 119 rather than an APD detector, as the detector for windmeasurement.

XWS and Weapon Alignments

In the XWS system, the aim point optical axis of the laser 123 and theoptical axis of each wind measurement channel 500 and 600 in FIGS. 5 and6 may be factory-aligned to ensure that the angular misalignment betweenthem is less than 50 micro radians. This may be done in the factory on acalibration fixture by bringing the laser spot reflected from a distanttarget (say 500 meters) to the center of the FOV of the color imagingsensor 120 and each of the wind signal sensors 113, 119. The electroniccrosshair, pre-aligned to the color imaging view finder center, may thenbe driven by pushbutton toggle switches to the center of the laser spotwhich is visible in the wind signal sensor 113, 119, and the coordinatesare set to (0,0) in the non-volatile memory. The parallax introduced isreduced by increasing the target distance or by knowing the offsetbetween the receiver and laser beam axes that aligns the laser beamparallel to the receiver axis. This factory zero alignment is achievableto better than 50 micro radians.

According to an exemplary embodiment, the factory alignment may includea “weapon zeroing” operation, illustrated as 2800 in FIG. 10, asfollows. If the XWS 100 is mounted directly to the rifle without adirect view optic (DVO) riflescope, the laser aim point may be zeroed tothe rifle bore so that the projectile impact point coincides with thelaser aim point. If the system is not integrated with the beam steeringmodule 107, the electronic crosshair 5001 (FIG. 19) may be used instead.The position of the electronic crosshair in the view finder is moved bythe toggle pushbutton switches to align with the center of the riflebore instead of moving the laser beam 130.

To find the center of the rifle bore, two methods may be utilized. Inthe first method, the user 207 commands the device 100 to enter a laserzero setup mode. This causes, in reference to FIG. 19, the zeroedelectronic crosshair 5001 to be displayed in the XWS view finder 5000.The electronic crosshair 5001 of the XWS 100 is placed at a fixed pointon a target 115 (FIG. 4) at a known distance (approximately 100 to 150meters (or less) to minimize any wind effects) and a series of shots arefired. The crosshair 5001 is then moved by pushbutton toggle switches sothat its center coincides with the center of the cluster of hits on thetarget 115. The process is repeated to improve weapon zero alignmentaccuracy. In the second method, a bore-sighting visible laser insertedinto the rifle bore could also be used, thus minimizing the expense offiring multiple shots. Once aligned, the new coordinates of theelectronic crosshair with respect to the weapon bore, which may bedifferent from the factory zero, are recorded in the memory, and thecrosshair is returned to the factory zero. Returning the crosshair tofactory zero guarantees the user the original position of the laser aimpoint angle. The difference in the coordinates determined from thealignments is used to correct the offset aim-point (OAP) 5007 that isdisplayed to the user in the wind and range measurement operations. Thisalignment procedure is called weapon zeroing, and remains valid untilthe XWS is removed from the weapon mount. This approach is preferredsince it can be done through the software, without requiring a laserbeam steering device such as Risleys or other mechanical means that addcost to the system. The only constraint of this approach is that itrequires the total misalignment (including the XWS mounting hardwarealignment to the rifle, the pic rail alignment to the rifle, and the XWSfactory zeroing of the laser to the receiver channels) to be smallcompared to the optical field of view. Alignment analysis shows thatthis may be much smaller than 0.5 degree compared to 3 to 5 degree fieldof view of the XWS 100 system.

When the XWS 100 is used with an existing riflescope as a piggybackdevice, the alignment process is the same as described above except theriflescope must first be boresighted with the rifle bore. This may bedone by firing a group of shots or using a boresighting laser insertedinto rifle bore. To display the crosshair and offset aim point in theriflescope sight, an optical display assembly (such as an OLED or LCOS)mounted in front of the riflescope may be used.

If beam steering module 106 is integrated with the XWS 100 and set tosteer equally in either direction, the misalignment tolerances may beloosened up by performing a “laser zeroing” allowing the laser line ofsight (LOS) in to be aligned with the weapon zero boresight. In thisprocedure, the user 207 commands the device 100 to enter a Laser Zerosetup mode, as illustrated in 2900 of FIG. 10. This causes, in referenceto FIG. 19, the zeroed electronic crosshair 5001 to be displayed in theXWS view 5000. Using the laser beam steering module 106 (FIG. 3), thelaser spot 116 is driven to coincide with the crosshair 5001 (FIG. 19).The procedure introduces parallax since the LOS and weapon zero isaligned at a finite distance ˜500 meters. Alternatively, the laser pathcould be made parallel to the weapon zero 5001 eliminating the parallax.

Once the laser aim point (i.e., laser LOS) or the electronic crosshairis aligned with the weapon zero 5001, a pushbutton switch may beactivated to cause the processor 1300 to redefine the co-ordinate systemzero to the current position of the electronic crosshair zero 5001. TheXWS 100 is then sighted-in. The laser aim point of the XWS 100 isproperly aligned to the rifle bore and its angular position known to theuser during operation.

Operating Procedure

In the preferred embodiment, the XWS 100 is mounted in place of theriflescope, and is used for target acquisition as well as wind/rangemeasurement. Once a target has been identified, the hunter places thecrosshair 5001 on the target and initiates a wind measurement 3300 (FIG.11) and a range measurement 3200 (FIG. 11). The required offset aimpoint icon 5007 appears on the system's digital screen 5000 displayed asan overlay with the target image as shown in FIG. 19. Once the OAP isdisplayed and the confidence indicator is about the size of the target,the user releases the measure-wind button 1000 to terminate themeasurement, and moves his rifle to position the center of the icon 5007on the target 115 he wishes to engage. Although shown as a cross, theaim point offset indicator 5007 could be programmed also as a square, acircle, etc. Likewise the size of the indicator can be modulated toindicate times when the uncertainty of the wind measurement is high. Theuncertainty being based on the variance of the wind readings over someset time interval.

Alternative Embodiment

In the above-described embodiments, the path-weighted average crosswindis calculated in the time domain, i.e., based on a covariance techniquewhich determines a time delay of the signal received by the dual windmeasurement channels 500, 600, to obtain parameters such as the offsetaim point (OAP) and the confidence metric. However, in an alternativeembodiment described herein below, a different method may be used tocalculate the path-weighted average crosswind, which is based onfrequency analysis of the time-varying scintillation fade signal. Thisalternative embodiment can be advantageous in that it only requires theuse of a single wind measurement channel By eliminating the need formultiple wind measurement channels, the size and cost of the system canbe reduced, which is desirable for both commercial and militaryapplications.

FIG. 20 is a block diagram of an XMS 100A according to an alternativeembodiment. It should be noted that various elements in FIG. 20 aresimilar in operation and structure as corresponding elements in FIG. 2,and thus are illustrated with similar reference numerals. Further, sincethese elements have already been described above, such descriptions willnot be repeated here.

However, as illustrated in FIG. 20, the XMS 100A differs from theprevious embodiments in that there is only one wind measurement channel(i.e., channel 500). As alluded to earlier, this wind measurementchannel 500 in FIG. 20 may have the same general structure and operationas previously-discussed embodiments, e.g., as illustrated in FIG. 5. Assuch, the wind measurement channel 500 includes an image sensor detector113, and this detector 113 may be implemented as a CMOS camera. Oneproperty of a CMOS camera is that it measures energy, and this may beadvantageous over other types of optical detectors which measure power(e.g., APDs (avalanche photodiodes)) due to better noise-limitingperformance. I.e., the limiting noise in a CMOS camera is approximately37 dB lower than power detectors, meaning that the CMOS camera requires37 dB less power for comparable performance. However, it is not strictlyrequired in this alternative embodiment that a CMOS camera be used asthe image sensor detector 113 of the wind measurement channel 500; othertypes of image sensors may also be used.

In FIG. 20, it is shown that the laser transmitter in the XMS 100A maybe configured as a dual divergence laser collimator 300A. Similar toprevious embodiments, this laser transmitter 300A may be configured tooperate the laser source in a CW (continuous wave) mode while measuringcrosswinds (and in pulsed mode for range measurements). However, thedual divergence laser collimator 300A is capable of creating a narrowbright spot (e.g., 100 μrad or less) on the target, while simultaneouslycreating a larger (and less bright) beam to illuminate the areasurrounding target (e.g., for nighttime use). For example, the dualdivergence laser beam may be generated using a diffractive optic element(DOE) with preselected power distribution. The narrow bright spot isintended for use in the crosswind measurements. It is contemplated thatthe narrow bright spot can be collimated on the order of 100 μrad by thelaser collimator 300A, thus allowing for a smaller-sized receiveraperture 102 in the wind measurement channel 500. This could beadvantageous in that there is a penalty for making the receiver aperture102 too large, that of increasing the system form factor thereby makingit too large for mounting on a rifle. It is contemplated that the sizeof the receiver aperture 102 should be limited to about 60 mm or less.For instance, the use of 35 mm receiver apertures may be advantageousfor rifle-mounted systems.

Further, unlike previous embodiments, the signal processor 1300A in theXMS 100A of FIG. 20 is programmed to measure a frequency of thescintillation fades obtained via a single wind measurement channel 500,and use this frequency to calculate a magnitude of the crosswind.According to the aforementioned article by Wang et al., the relationshipbetween the crosswind magnitude and the frequency of scintillationsignal can be represented by the following expression:

Crosswind=m(Cn ² k ² L)^(−0.6) d  Eq. (1)

where m is a calibration constant of the system, Cn² is a refractiveindex structure parameter, k is the wave number of the laser, L is therange to the target, and f is the frequency of the scintillation fades.This equation is derived for weak scintillation conditions (i.e.,Cn²<10⁻⁵) and does not include the effects from aperture averaging.Therefore, the crosswinds when estimated are subject to errors as theturbulence strength increases and approaches saturation (Cn²>10⁻¹³). Thecalibration constant m, i.e., the parameter in Eq. (1) which relates thewind to the scintillation frequency, changes significantly due to thepresence of smaller eddies as the turbulence strength increases. Forweak turbulence, however, the calibration constant m stays relativelyconstant.

Previous systems have attempted to solve the problem of wind measurementerrors at saturated scintillation due to changes to the calibrationconstant (see U.S. Pat. No. 7,739,823 B2, the entire contents of whichare incorporated herein by reference), but rely on an incoherent sourceof a relatively large size (300 μrad) and a large receiver diameter (80mm).

However, the XMS 100A of FIG. 20 is able reduce errors in crosswindmeasurement due to the aforementioned changes to the calibrationconstant m, through the use of a CW laser source (rather than a pulsedsource) in combination with a CMOS camera. This allows the XMS 100A toreduce such errors to less than 5% over a wide range of scintillationconditions (i.e., Cn² in the range of 5×10⁻¹³ to 10⁻¹⁶), with a suitablysmall laser spot size (e.g., 100 μrad) and receiver aperture (e.g., 35mm). Particularly, in the XMS 100A, the CMOS camera measures thescintillation fluctuations at the frame rate of the camera. This meansthat scintillation signal is integrated over the time at which eachframe is captured. Accordingly, under saturated conditions, anyfrequencies of the scintillation pattern which is higher that the framerate are averaged out by the integration for each frame, and thus do notadd noise to the measurement.

In summary, similar to previous embodiments described above, the XMS100A of FIG. 20 may be implemented as a system package including anoptical transmitter (300A), an optical receiver and a controllerprocessor (1300A) in combination with other elements such as a displaydevice and a suite of sensors for parameters that affect the ballistics,and possibly additional features, e.g., a local bi-directionalcommunication link to an external device. Unlike previous embodiments,this optical receiver may be configured to include a single windmeasurement channel 500 to measure the crosswind, i.e., by detectinglight reflected from the target which travels back toward the systemthrough atmospheric turbulence eddies that move with crosswinds.Particularly, an image sensor 113 (e.g., CMOS camera) may detect suchlight via the wind measurement channel 500 to provide a time varyingatmospheric scintillation signal, which may be processed by theprocessor 1300A in the frequency domain to determine a path-weightedaverage crosswind. Furthermore, the processor 1300A may obtain repeatedmeasurements of the aforementioned frequency based on the detectionresults of the wind measurement channel and, based on these repeatedmeasurements, calculate respective crosswind values and average themtogether to obtain the path-weighted average crosswind. Also, theprocessor 1300A may be programmed to calculate a confidence metricrepresenting the accuracy of the determined path-weighted averagecrosswind based on the variance of the repeated measurements.

Furthermore, the system package of FIG. 20 may be mounted to a weaponsuch as a rifle, gun, archer's bow, or crossbow to measure path-weightedaverage crosswinds from the shooter to the target, range to target,environmental parameters to generate a full ballistic offset solutionuseful to the shooter to repoint the weapon. Also, in accordance with afurther embodiment which will be described in more detail below, such asystem package may be adapted for use with an external riflescope, i.e.,by overlaying generated data over top of the optically-produced imageobtained by the riflescope.

Now, reference will be made to FIG. 21 which illustrates a flowchart ofa process 4000 for determining crosswinds based on a frequency ofscintillation fade. Particularly, this process 4000 of FIG. 21determines the crosswind magnitude based on the relationship expressedin Eq. (1), and thus utilizes parameters including the scintillationfade frequency f, the refractive index structure parameter Cn², therange to the target L, the calibration constant m, and the laser wavenumber k.

The parameters m and k of Eq. (1) are known prior to each crosswindmeasurement performed in accordance with process 4000. Range L and Cn²parameter will not be constant, so they will be measured for each windmeasurement according to process 4000 The laser wavenumber k is a fixedvalue, i.e., a known wavelength design parameter. The calibrationconstant m, on the other hand, can be determined by conducting a fieldexperiment under controlled conditions, i.e., in which the range L andrefractive index structure parameter Cn² are known and kept constant. Bydesign, the wind measurement receiver aperture and laser spot size arechosen to minimize the effect of scintillation saturation on thecalibration constant m, so that this parameter m can be assumed toremain constant without significant errors in wind measurement.

A field experiment for determining the calibration constant m for theXMS 100A will now be described in more detail. In such an experiment, arifle can be aimed at a fixed point on a target at a distance of L, andthe horizontal deflection of the bullet due to crosswinds can bemeasured as the deviation of the hit point from the target or aim point.A number of shots may be fired under varying wind conditions, and dataregarding the bullet deviation versus the detected scintillation signalsfor each of the shots may be collected. For instance, the experimental“ballistic crosswind” for each shot can be obtained using a ballisticcalculator and the measured bullet deviation for that bullet type. Themeasured bullet deviation can thus be used to determine thepath-averaged crosswind. The detected scintillation signals may beconverted into a frequency for each shot, e.g., in a similar manner aswill be described below in connection with process 4000. FIG. 25, whichillustrates an example in which the data of the ballistic wind versusthe measured frequency are plotted, shows a linear relationship betweenthe ballistic wind and the scintillation fade frequency f. It is assumedthat the slope of this line equals the expression m(Cn²k²L)^(−0.6) inEq. (1), which thus allows determination of the calibration constant min view of the calculated frequency f, since the other parameters(wavelength k, range L, and refractive index structure parameter Cn²)are fixed and known. It should be noted that, during normal use of theXMS 100A, the range L and Cn² parameter will not be constant, so theywill be measured for each wind measurement according to process 4000,along with the scintillation frequency f, and used along with thepreviously-determined system calibration constant m to calculate thecrosswind magnitude.

Referring again to FIG. 21, the process 4000 starts at S4010 in which,e.g., the user may trigger the crosswind measurement operation bypressing a button or similar operation. As illustrated in S4015, therange L is measured using the ranging receiver 400. As described above,the range measurement may be performed by operating a laser in pulsedmode. Once the range is determined with desired accuracy, the laser maybe switched to CW mode in order to detect the scintillation patternaccording to S4020. In this operation S4020, the CW laser may be turnedon for a fixed time tw, during which the scintillation patterns aredetected by an image sensor detector 113 (which for purposes of thisdiscussion is assumed to be a CMOS camera). Particularly, the CMOScamera 113 will capture a certain number of image frames depending onthe camera's frame rate. In each of these image frames, the CMOS camera113 will be detecting the light which reflects off of the target andtravels back toward the wind measurement channel 500 through atmosphericturbulence eddies, such eddies moving with the crosswinds.

As shown in operation S4025 of FIG. 21, the value of the refractiveindex structure parameter Cn² for each crosswind measurement is obtainedin accordance with process 4000. The value of Cn² may be determined fromthe set of images detected by the CMOS camera 113 during the fixed timetw, e.g., by tracking the centroid of the laser spot as a function oftime, and computing the variance of the centroid position.

Thereafter, in S4030, the frequency f of the scintillation fluctuationsare calculated based on the scintillation signals or patternsrepresented in the frames captured during the fixed time tw. Thisfrequency f can be calculated in one of several ways, including: (1) azero-crossing approach, which obtains a time-varying scintillationsignal from the frames and counts the number of times the signal crossesthe mean value per unit time; (2) using a Fast Fourier Transform; and(3) calculating the half-width of the auto-covariance function, which isinverse to the frequency. Here, it will be assumed that option (1),i.e., the zero-crossing approach, will be used to calculate thefrequency f in S4030. A more detailed explanation of this approach willbe provided below in connection with FIG. 22.

Particularly, FIG. 22 illustrates a specific process 4100, which may beapplied in operation S4030 of FIG. 21 to calculate the scintillationfade frequency f. After this process 4100 is initiated (S4110), theseries of scintillation signals obtained during tw are processed inS4120 & S4130 in order to generate a time-varying scintillation signal.

As shown in FIG. 22, the time-varying scintillation signal can begenerated by first summing, in each frame, the laser intensity from allcamera pixels which are illuminated by the laser spot (operation S4120).Based on these calculations, the fluctuations of these intensities canbe plotted versus time, over the span of tw, according to S4130. Thisoperation S4130 results in a time-varying signal representing the energyfluctuations caused by scintillation of the laser spot. This signal maybe considered a “raw scintillation signal,” an example of which isillustrated in FIG. 24A. Particularly, FIG. 24A illustrates a typicalraw scintillation signal which can be obtained, using a CMOS camera 113which is able to acquire approximately 1000 frames over a time tw of atleast one second, from a range of approximately 700 meters. FIG. 24Aillustrates a particular example where the raw scintillation signalcomprises a mix of frequencies ranging from zero to one-half of theframe rate frequency of the receiver camera 113.

After the raw scintillation signal is obtained (S4130), the signal maybe further be processed according to operation S4140 in FIG. 24A. As tothe lower frequencies, including zero (DC), these frequencies do notgenerate enough cycles within a one-second period to be statisticallysignificant. Thus, in S4140, a digital low-pass filter may be applied tothe raw scintillation signal to filter out the low frequency component.FIG. 24B illustrates an example of the results of the low frequencycomponent that is filtered out from the signal of FIG. 24A. Thereafter,in S4140, the low-frequency component (FIG. 24B) is subtracted from theoriginal raw scintillation signal (FIG. 24A). This results in a versionof the time-varying scintillation signal, as shown in FIG. 24C, fromwhich the frequency f can be determined. Operation S4150 calculates thefrequency f from the signal obtained by S4140. Specifically, thisoperation S4150 counts the number of times the signal crosses zero, andcalculates the frequency f as half of the number of zero-crossings persecond. Referring back to process 4000 in FIG. 21, after the frequency fhas been measured (S4030), the crosswind magnitude can be calculated inS4035 by plugging the measured parameters (L, Cn², and f) into Eq. (1).Further, as shown in S4040, the processor 1300A may repeatedly measurethe frequency f according the detection results of the wind measurementchannel 500 (i.e., by repeating operations S4020 & S4030). The purposeof these repeated measurements would be to calculate respectivecrosswind values and average them together (see S4045) to obtain thepath-weighted average crosswind. Also, the processor 1300A may bedesigned to calculate a confidence metric representing the accuracy ofthis path-weighted average crosswind, i.e., by calculating the varianceof the repeated measurements.

It is also possible, as part of these repeated measurements, torepeatedly calculate the Cn² parameter (S4025). However, as illustratedby the dotted line connection S4020 and S4030, it is also possible toavoid the repeated measurement of Cn² in addition to the frequency f ifdesired.

After the crosswind magnitude(s) is calculated (and averaged together),S4045 also includes an operation of determining the crosswind direction,which may be necessary in order to estimate the offset aim point. FIG.23 illustrates an exemplary process 4200 for determining the crosswinddirection based on the detection results obtained via the windmeasurement channel 500. After the process 4200 is initiated (S4210),according to S4220, the laser spot image in each of the captured framesis divided into two groups of pixels, e.g., half to the right of thevertical center line and half to the left. Then, in S4230, the laserspot intensity from each of the halves is summed to yield twotime-varying signals (left-half and right-half). According to S4240, across-covariance of these two signals is calculated. The location of thepeak of the cross-covariance, with respect to the zero time reference,provides the wind direction. If the peak is on the negative side, thewind direction is from left to right; if the peak is on the positiveside, wind direction is from right to left. This determination is madeaccording to S4250.

Returning to FIG. 21, after the magnitude and direction of the crosswindis obtained, the results can be displayed to the user, along with otherinformation (e.g., offset aim point and confidence metric) derived fromthe crosswind magnitude and direction (S4050).

In this alternative embodiment, which is described above in connectionwith FIGS. 20-25, the crosswind measurement is made possible bycalculating the frequency f of a scintillation pattern, which increasesalong with the crosswind. For the system 100A to provide highly accuratecrosswind measurements, the frame rate of the image sensor detector 113(e.g., CMOS camera) should be greater than the scintillation fadefrequency f. For example, if the crosswinds are measured to 30 mph, itwould be preferable for the frame rate of the camera 113 to be in excessof 2000 Hz—this would make it possible to achieve an accuracy of 0.5 m/sfor hitting a target in a 50 cm circle at 1000 meters, with aprobability greater than 60% probability. Of course, lower frame ratesmight degrade the accuracy.

It should be noted that the flowcharts of FIGS. 21-23 are provided forpurpose of example only, and are not intended to be limiting. Forinstance, the order of operations in these figures may be changed, someoperations may be omitted and others may be added to the underlyingprocesses.

Further Embodiment

According to a further embodiment, the XMS system 100, 100A in the aboveembodiments can be specially configured for use with existingriflescopes, thereby allowing the user to retain the direct view optics(DVO) which do not rely on battery power. An example of a system 100Baccording to the further embodiment is illustrated in FIG. 26.Particularly, this XMS 100B is illustrated as a modification to thealternative embodiment system 100A utilizing a single wind measurementchannel 500. Accordingly, the system 100B may also include a dualdivergence laser collimator 300A of the above-described alternativeembodiment. However, the principles of this further embodiment can alsobe applied to embodiments of an XMS 100 utilizing two wind measurementchannels 500, 600, e.g., by replacing the eyepiece 105 with thesymbology projector 200 which is described in more detail below.

As illustrated in FIG. 26 (described in more detail below) the system100B is mounted on an existing picatinny rail 9000 on which the externalriflescope 8000 is also mounted. Particularly, the system 100B (or atleast some components thereof) may be mounted in-line with theriflescope 8000 on the picatinny rail 9000, rather than being mounted ontop of the riflescope 8000 so as to minimize obscurations of the fieldof view.

Similar to previous embodiments, the XMS 100B in the further embodimentcan be used by illuminating a target with a laser beam, measuring therange and crosswinds, and calculating and displaying the ballisticsolution (including, e.g., offset aim point (OAP), average winds,azimuth and elevation offsets, and range to target). However, theballistic solution information can be displayed using a built-insymbology projector 200, which overlays the information overtop the DVOimage, e.g., as symbols, numerals and/or alphabetic characters. Thisoverlaid information may prompt the user to re-aim the weapon byaligning the OAP on the target image, instead of the original crosshairposition. This feature has an advantage in that it allows the user tokeep the eyes on the target during engagements. Because the symbologyprojector 200 is designed to be located in-line and in front of theriflescope objective lens, it does not degrade the optical imagingresolution (modulation transfer function (MTF)) or introduce blur oraffect the original twilight factor of the riflescope.

Particularly, as information regarding the ballistic solution (OAP,confidence metric, range to target and/or other symbology) is createdand activated by the processor 1300B of FIG. 26, and sent to thesymbology projector 200. As such, the symbology projector 200 isdesigned to project the received information, in real-time, onto the DVOof the riflescope. To do this, the symbology projector 200 may include aLiquid Crystal On Silicon (LCOS) chip or Organic Light Emitting Diode(OLED), along with lenses and prisms for the symbology projection to theriflescope object plane.

FIG. 26 is a functional block diagram of the XMS 100B. As describedabove, the system includes a single wind measurement channel 500B foruse, e.g., in implementing the above-described zero crossing method formeasuring crosswinds. However, as mentioned above, the principles ofthis further embodiment could also be applied to adapt the XMS 100illustrated in FIG. 2. Since the system 100B of FIG. 26 is designed foruse with an existing riflescope 8000, the integrated electronic eyepiece105 of FIG. 2 would not be used, but instead replaced with theintegrated symbology projector 200. In operation, the shooter views thetarget through the riflescope 8000 of FIG. 26. The crosswind measurementis performed as described above, but the results are projected onto theriflescope 8000 via the symbology projector 200 as an overlay on thetarget image showing “critical information,” e.g., range to target, thepath-averaged crosswind in the path toward the target, azimuth andelevation numeric offsets, confidence metric, and OPA. For instance, theOAP may be represented by an overlaid circle, square or diamond that theuser uses to re-point the weapon.

FIG. 27 illustrates a more detailed block diagram of the symbologyprojector 200. As shown in this figure, the symbology projector 200 mayuse a pixelated LCOS chip 210, illuminated by an LED beam at 600-650 nmwith a collimating lens 220, and a diffuser/wire-grid polarizer 230which linearly polarizes the light. In FIG. 27, the light enters apolarizing beam splitter (PBS) cube 240 illuminating the LCOS 210 withS-polarized light. This light reflects off the LCOS 210, turning thelinear polarization vector by 90 degrees. The light from any given pointon the LCOS 210 is divergent. This diverging light, which isP-polarized, reenters the PBS cube 240 and transmits through it. Thelight then passes through a weakly-powered focus lens 250. The focuslens can move in the optical axis direction for parallax adjustment.After passing through the focus lens, the light reflects off a foldmirror 260 and enters a telephoto lens. The telephoto lens consists of anegative lens element 270 which expands the light and a positive lenselement 280 which collimates the light. The light then reflects off abroadband polarizer coating on the beam-splitter plate 104 and entersthe riflescope 8000.

The projected image of the LCOS chip 210 includes lit-up pixels with thesymbology of the ballistic information. These pixels (i.e., symbology)are presented to the user in the field of view of the riflescope 8000.Because image projected is in the object plane, the effect of riflescopezooming, which changes the size of the riflescope reticle, tracks withthe projected offset aim point, confidence indicator and other symbologydisplayed. Visible light from the target passes through and around thesymbology projector's 200 beam splitter 104.

Second Alternative Embodiment

The embodiment first described in this application (referred tohereafter as “first embodiment”) utilizes a cross-covariance time delayto calculate the path-weighted average crosswind. According to the firstembodiment, the calculated crosswind can be processed by a ballisticcalculator, using the measured range and environmental and ammunitionparameters, to provide an offset aim point (OAP) and confidence metricwhereby a user can repoint the weapon to hit targets accurately andefficiently with a first round. As described above, the first embodimentuses a pair of side-by-side horizontally-separated receiver channels 500and 600, thus incorporating two apertures 102 and 103 (objective lenses)and two detectors 113 and 119 (digital imagers or PIN or APDphotodiodes) to collect the reflected laser light from a distant laserspot illuminated by the user. In the first embodiment, each receiverchannel 500, 600 provides a time varying signal due to the intensitychanges caused by atmospheric turbulent eddies through which thereflected laser beam from the target traveled back. This time delaybetween the two returned optical paths from the laser spot 116 isgoverned by the time required for the wind to blow atmospheric eddiesfrom one optical path into the other. In the first embodiment, this timedelay is determined by conducting a covariance of the two time-varyingsignals which yielded a displaced peak in time if the two signals arewithin the atmospheric frozen flow time. The ratio of the effectivehorizontal separation between the receivers 500, 600 and the measuredtime delay results in the path integrated crosswinds.

An alternative embodiment is also described above, in which thepath-weighted average crosswind is calculated based on a frequencyanalysis of the scintillation fade signal, instead of the time domaincalculations of the first embodiment, thereby foregoing the need for twooptical apertures (and two corresponding channels) to measure thecrosswind. Accordingly, as illustrated in FIG. 20, only one windmeasurement channel 500 needs to be provided in the XMS 100A of thisalternative embodiment.

Now, a second alternative embodiment will be described herein belowusing the pupil plane imaging of the first embodiment, which alsoeliminates the need of a second aperture to measure the path-weightedaveraged crosswind. As such, the second alternative embodiment is alsocompatible with the XMS 100A illustrated in FIG. 20, under an opticalconfiguration in which the wind measurement channel 500 functionsvirtually as two separate channels required for crosswind determination.

Similar to the first embodiment, the second alternative embodimentdescribed hereinbelow is capable of measuring downrange path-integratedcrosswinds by determining a time delay between two time-varying signals.However, these two signals are obtained via a single optical aperture,rather than the two apertures used in the first embodiment. Despite thisdifference, the second alternative embodiment is still compatible with aboresighting alignment procedure similar to the one described above.

Relevant Aspects of First Embodiment:

For purposes of comparing the first embodiment with the secondalternative embodiment, reference will now be made to FIG. 28 todescribe some basic principles of the first embodiment. Specifically,FIG. 28 provides a simplified illustration of the use of two aperturesand two corresponding wind measurement channels A and B in accordancewith the first embodiment. As shown in FIG. 28, the reflected laserlight incident on the objective lens of channel A travels through PATHA, while the reflected laser light incident on the objective lens ofchannel B travels through PATH B. Each objective lens focuses thereflected light onto a detector, which can be a CMOS image sensor asillustrated in FIG. 28, or else an APD or a PIN photodiode. If thedetector is an image sensor, the power of the reflected spot from thetarget would be distributed over several pixels of the sensor determinedby diffraction. In this case, in each of channels A and B, the detectedpixel values of the reflected spot would be summed together to yield thesignals P_(A)(t) and P_(B)(t). These time-varying signals P_(A)(t) andP_(B)(t) are processed to obtain the crosswind measurement. Each ofthese signals P_(A)(t) and P_(B)(t) is representative of the total laserlight, which has been reflected from the illuminated spot on the targetand is incident on the respective receiver aperture, at a given time.

Reducing Hardware in Second Alternative Embodiment:

However, the second alternative embodiment can generate two similartime-varying signals through the use of a single aperture as illustratedin FIG. 30 (which will be described below), rather than the twoapertures illustrated in FIG. 28. Basically, the second alternativeembodiment is able to retain the functionality of two wind measurementchannels, using only the hardware associated with a single windmeasurement channel of the first embodiment and eliminating the need forthe second wind measurement channel. This is useful in reducing thehardware components, reduce cost and simplify the complexity of opticalalignment process.

Correcting for Environmental Conditions:

Both the first embodiment and the second alternative embodiment can beimplemented in such manner as to increase the accuracy of crosswindmeasurements in view of changes to atmospheric conditions that mightotherwise degrade accuracy. Particularly, in algorithms for measuring atime delay between two signals (such as the Brigg's and Peak methodsdescribed in Wang et al., “Wind measurements by the temporalcross-correlation of the optical scintillations,” Applied Optics V20,No. 23, December 1981), it has been observed that changes in atmosphericstructure constant Cn² and higher wind speeds (8 to 10 mph or higher)can affect the accuracy of the crosswind measurement. It is possible tocarry out the Peak or Brigg's method under the assumption that the timedelay tp (Peak) or t_(c) (Brigg's) is insensitive to environmentaleffects, in the manner described above in connection with the firstembodiment. Under this assumption, the crosswind would be determinedusing the measured time delay, and the integral of a path-averagedproportionality factor function of the two receivers' return paths,which is representative of an effective receiver separation D_(eff). Forexample, in the Peak method, the time delay tp and the effectivereceiver separation D_(eff) would be related to the crosswind speed Vaccording to the equation V=D_(eff)/tp.

It should be noted that, for purposes of describing the secondalternative embodiment, the symbol V will be used from now on torepresent the crosswind speed, instead of the symbol W which was used inconnection with the first embodiment. The intent for changing the symbolof crosswind speed is to avoid confusion with a path weighting functiondesignated with the symbol W, which will be discussed below.

The effective receiver separation D_(eff) determines the calibration ofthe system, and is dependent on the laser spot size D_(T), at the target115 as well as the following known parameters which have already beendescribed above:

-   -   Value of refractive index structure constant Cn²    -   Magnitude of crosswind speed V    -   Range to target L    -   Physical horizontal separation between the two receiver        apertures p    -   Wavelength of the laser λ    -   Divergence of the laser beam θ

Of the above parameters ρ, λ, and θ are design constants; and L can bemeasured prior to each crosswind measurement. The remaining parameters,Cn² and V, change with environmental conditions and time. However, beingthat changes in Cn² and V have been observed to change the laser spotsize D_(T) (which had previously been calculated based on parameters Land θ), it is evident that in certain situations it might be beneficialto correct the value of the effective receiver separation D_(eff) tocompensate for the changes in Cn² and V, in order to minimize errors incrosswind measurements. This potential benefit has been observed inmeasured field conditions, as well as data which has been recordedshowing the impact of the environmental changes on the accuracy of windmeasurements.

Specifically, data has shown that the effect of changes in Cn² andcrosswind speed V on the size of the laser spot D_(T) is negligibleunder low to moderate values (up to 10⁻¹⁴) for Cn². This is illustratedin the graph of FIG. 29, which correlates changes in the refractiveindex structure constant Cn² to the measured laser spot size. As shownin FIG. 29, the measured changes in the spot size is relatively flat asCn² increases to about 3×10⁻¹⁴, thus producing an acceptable crosswinderror on the order of +1-5%. However, as Cn² increases beyond 3×10⁻¹⁴,the laser spot size increases substantially, thus meaning that theD_(eff) parameter could be compensated to obtain a more accurateestimation of the crosswind. Otherwise, an uncorrected value of theD_(eff) could have an impact of introducing larger errors in themeasured crosswind V thus resulting in target misses.

Data has also shown that a similar curve to those shown in FIG. 29correlates the laser spot size D_(T) changes to increase in crosswinds.A flat response has been measured with winds of up to 8 to 10 mph.However, at increased wind speeds, it might be advantageous to correctD_(eff) to provide a more accurate calculation of the crosswind speed V.Regardless of which parameter changes due to environmental impacts, Cn²or V or both, the effect is a change in the laser spot size D_(T) whichgoverns the value of D_(eff).

It should be noted that the corrections to the value of D_(eff) asdescribed hereinbelow could be applicable and beneficial not only to thesecond alternative embodiment described herein, but also to the firstembodiment described in this specification.

Structural Configuration of Second Alternative Embodiment:

FIG. 30 is a diagram illustrating the configuration of a receiverutilizing a single optical aperture to generate two time-varying signalsfor measuring a time delay and corresponding crosswind, in accordancewith the second alternative embodiment. Basically, the structure in FIG.30 represents a modification of the wind measurement channel 500 ofprevious embodiments, and thus is labeled as 500′ in the figure. Asshown in FIG. 30, the laser light, which is reflected from the distanttarget 115, enters through a single optical receiver aperture, i.e.,objective lens 102. The objective lens 102 is shown in FIG. 30 ascollecting the reflected laser light as nearly-collimated, and focusingan image of the target spot at its focal point f1. As shown in FIG. 30,a field stop FS may be provided at a distance away from the objectivelens 102 equivalent to the focal length f1 of the objective lens 102.The purpose of positioning the field stop FS at this location is tominimize the amount of stray light from around the target 115 that iscollected into the imaged signals. From this focused point at field stopFS, the light then diverges and is collected by reimaging optics RO. Forpurposes of illustration, the reimaging optics RO is illustrated as asingle lens (i.e., a secondary lens) in FIG. 30, which is positioned adistance away from the field stop FS equivalent to the lens's focallength 12. However, the reimaging optics RO need not be composed of asingle lens, but instead may comprise a combination of lenses and/oroptical elements (as will be described below in connection with FIG.31). In FIG. 30, the output of the reimaging optics RO is sent to thesignal detector 113 (e.g., digital image sensor, APD or PIN photodiode)as an inverted and de-magnified image of the light entering the pupil ofthe objective lens 102. Although FIG. 30 illustrates the output of thereimaging optics RO as a collimated beam, this assumes that the focalpoint of the incoming light coincides precisely with the focal length 12of the secondary lens, which may not always be the case. As such, it iscontemplated that the beam outputted by the secondary lens of FIG. 30could alternatively be slightly converging or diverging. As to the imageobtained by the detector 113, the pixels of this circular image can besorted into two images, one comprised of the pixels from the left half,the other comprised of the pixels from the right half. As such, theleft-half and right-half images from the detector 113 are used,respectively, to obtain two separate time-varying signals as will bedescribed below. Thus, even though the arrangement of FIG. 30 physicallyconstitutes a single aperture channel 500′, it effectively functions astwo optical channels for measuring the crosswind.

Since, according to the configuration of FIG. 30, the detector 113 isimaging the light entering the pupil itself, instead of the focusedimage of the target spot (at the focal length f1 from the objective lens102), this configuration is employing what is referred to as a “pupilplane imaging” to measuring crosswind as described in the firstembodiment.

As mentioned above, while the reimaging optics RO is shown as outputtinga collimated beam to the detector 113 in FIG. 30, the reimaging opticsRO could also produce a slightly converging or diverging beam (as willbe described below in connection with FIG. 31). The important criteriafor the reimaging optics RO is that they will cause the entrance pupilof the objective lens 102 to be imaged on the detector 113 that retainsthe physical relationship of light collected by first or second half ofthe objective lens to the second or first half of the pupil plane imagedspot on the detector 113.

Referring again to FIG. 30, assuming that an image sensor is implementedas the detector 113, the de-magnification properties of the reimagingoptics RO are determined by the required signal to noise ratio (SNR) perpixel, as well as the pixel size, of the image sensor 113. For example,an optical link budget analysis has shown that, when the image sensorhas 15 micron pixels, in combination with a laser beam divergence of 100micro-radians, an objective lens 102 of diameter 50 mm, and a target 115that is 1 km away, shows that the input aperture beam could bede-magnified to a 315 micron collimated beam illuminating approximately21 pixels of the image sensor. In this example, the input beam size isreduced to 315 micron (a factor of approximately 159) by proper designof the reimaging optics RO.

By summing the response of each of the pixels in the left half of thecircular image spot, obtained by the image sensor 113, a signal P_(A)(t)can be obtained that is equivalent to the total power entering the righthalf of the objective lens 102. Likewise, summing the pixel responsesfrom the right half of the image spot at the image sensor 113 yields atotal power P_(B)(t) entering the left half of the objective lens 102.It is noted that a central column of the imaged entrance pupil, locatedat a dividing line (pixel column) of the image sensor 113 relative tothe left and right halves of the image, may receive signals from bothsides. This is referred to as “crosstalk” between the left-half andright-half channels. This crosstalk can be rejected simply by filteringout the central portion of the imaged pupil, e.g., by including theoverlap pixels of this central column in the summations of both theleft-half signal P_(A)(t) and the right-half signal P_(B)(t). Since thetotal illuminated area in this example covers 346 pixels, and thecentral column that is filtered out has 21 pixels the filtering resultsin a 6% reduction in signal level.

The arrangement of FIG. 30 provides the same information regarding thecrosswind measurements as that described above in the first embodimentwhich utilizes two apertures separated by for example 50 mm, except thatthe separation of the two halves in FIG. 30 is reduced from 50 mm to 25mm. Accordingly, the image sensor (detector 113) may be sampled at aframe rate at least twice as fast as that used in the first embodimentconfiguration to maintain signal processing timing and accuracy ofmeasurement.

As mentioned above, the second alternative embodiment reduces thecomplexity and cost of the optical design, relative to the firstembodiment, while still using the same overall processing approach asthe first embodiment in calculating the crosswind (i.e., according toany of the processing methods discussed above in connection with FIG.12). This means that the single optical receiver aperture 102 of FIG. 30is able to generate the two time-varying signals required to measure atime delay according to the principles described above (e.g., using thePeak or Brigg's processing method). In addition, by eliminating the needfor the second horizontally-separated wind measurement channel 600described above, the second alternative embodiment can reduce the sizeand weight of the first embodiment XMS system and also simplify theoptical alignment.

As mentioned above, the single aperture approach described above inconnection with FIG. 30 does not image the target spot, but ratherimages the entrance pupil of the objective lens 102. While FIG. 30illustrates a collimated beam being transmitted from the reimagingoptics RO to the detector 113, such beam could alternatively beconverging or diverging as long as it is the entrance pupil of theobjective lens 102 that is being imaged by the detector 113. This singleaperture approach helps prevent crosstalk between the two halves of thespot, while still providing the same information (i.e., P_(A)(t) andP_(A)(t)) as the two-aperture configuration for crosswind measurementdescribed in the first embodiment (and FIG. 28).

However, if the single aperture structure of FIG. 30 were only to applypupil plane imaging as described above, this may pose constraints onother potential functions of the optical receiver that includeboresighting of the XMS system to the riflescope crosshairs, andmeasuring the laser spot size D_(T) to compensate for environmentalconditions. Thus, it would be advantageous to adapt the single aperturestructure of FIG. 30 to generate a focused spot onto a different area ofthe detector or image sensor 113. In other words, it would beadvantageous to adapt the single aperture structure so that one portionof the image sensor 113 images the laser spot on the target 115 (i.e.,performs “field imaging”) while another portion images the entrancepupil of the objective lens 102 b (i.e., performs pupil plane imaging).This can be achieved by incorporating a beam splitter to split theincoming light from the target 115 into two beams: one beam that isfocused directly onto a diffraction-limited spot on the image sensor 113(thus providing the field imaging to be used for boresighting and formeasuring the laser spot size D_(T)), and another beam that iscollimated by a lens PL and sent to a different area of the image sensor113 (thus providing the pupil plane imaging to be used for crosswindmeasurement).

FIG. 31 is a diagram illustrating a particular optical arrangement forutilizing a beam splitter to adapt the single aperture structure of FIG.30 to provide two optical paths: one that is de-magnified for pupilplane imaging (i.e., measuring crosswind), and the other for fieldimaging that focuses the laser spot directly onto the image sensor 113.Thus, while FIG. 31 illustrates a single aperture channel 500′ formeasuring crosswinds similar to FIG. 30, the structure in FIG. 31 isalso suitable for boresighting and measuring the laser spot size via itsfield imaging path. It is contemplated that the field imaging path maybe configured to provide an overall accuracy of 50 micro-radians, whichis a suitable accuracy for boresighting alignment and for measuring thelaser spot size D_(T) for compensating for environmental factors.

In FIG. 31, an objective lens 102 is placed at the entrance pupil. Forexample, this objective lens 102 may be configured with a diameter of 50mm and a focal length of 120 mm. Specifically, the objective lens 102collects light that is reflected from the laser-illuminated target 115and focuses it in such manner as to form an image of the field 120 mmaway (i.e., at the field stop FS). In the particular implementationshown in FIG. 31, the objective lens 102 actually consists of twooptical elements in order to athermalize its performance, thuspreventing the image location from changing as the system temperaturechanges. The field stop FS is placed at the location of the focal pointof the objective lens 102 to block unwanted light from sources aroundthe target 115. For instance, the field stop FS may be 0.72 mm indiameter, allowing for a 6 milli-radian field of view.

In FIG. 31, the light passes through the field stop FS to the reimagingoptics RO. In the particular implementation of FIG. 31, the reimagingoptics RO include a dual-element lens DE. This dual-element lens DEconsists of two elements having a diameter of, e.g., 5 mm andcollectively having a focal length of, e.g., 5 mm With such a diameterand focal length, the dual-element lens DE can reimage the scene ontothe image sensor 113 in such manner as to give the system an effectivefocal length of 300 mm, while requiring only 150 mm of physical space.In turn, this would provide for a 50 micro-radian instantaneous field ofview onto each 15 micron pixel of the image sensor 113. The two opticalelements in the dual-element lens DE can be made identical in order toreduce cost and complexity, and the two elements can be athermalized inorder to prevent the field image from moving in and out of focus on theimage sensor 113 as the temperature changes from −20° C. to 70° C.,which is typical for field-use applications.

In FIG. 31, next to the dual-element lens DE, the reimaging optics ROincludes a sun filter SF, i.e., a narrowband pass filter (ofapproximately 20 nm) which rejects light at wavelengths other than thatof the laser transmitter 123, thus suppressing background noise in thefield imaging and pupil plane imaging paths.

Also, as part of the reimaging optics RO, a beam-splitting opticalcomponent BS placed between the sun filter SF and the image sensor 113,dividing the light into two separate optical paths. One of these opticalpaths is for focusing the field image onto the image sensor 113, andthus provides a channel for purposes of boresighting and measuring thelaser spot size D_(T). The other optical path is for providing an imageof the entrance pupil of the objective lens 102 at the image sensor 113.The left and right halves of this pupil plane image are effectively usedas two channels from which the crosswind speed V can be measured.

The fraction of the light required for boresighting and measuring thesize of laser spot D_(T) is much less than 5%, because these functionsare generally carried out by the user at the start of each engagementwith an image sensor 113 at a low frame rate. Low frame rate captureassures a statistically significant sampling of the image motion with anintegration time of 1 second. The lower frame rate provides longerexposure time and lower noise floor than used in the crosswindmeasurement mode. For instance, in the crosswind measurement mode, itmay be appropriate to use a frame rate on the order of 2000 Hz, whileonly a 30 Hz frame rate may be sufficient for the boresighting and spotmeasurement mode. Under these conditions, the ratio of exposure timesfor the two modes can be approximately 66. Thus, the fraction of theinput signal to be split off for the field imaging path for boresightingand spot measurement can be as low as 1.5%, which has a negligibleimpact on the SNR needed to perform crosswind measurement.

To be safe, the beam splitter BS of FIG. 31 may be configured to allow5% of the light entering therein to pass through to the field imagingpath, to be focused on the image sensor 113. The other 95% of the lightis directed by the beam splitter BS onto a parallel path for crosswindmeasurement which is nearly collimated on the image sensor 113.

FIG. 31 provides an enlarged view of this beam splitter component 31 aswell as the two optical paths produced therefrom. As shown in FIG. 31,the beam-splitter BS may consist of a rhomboid prism and a right-angleprism which are bonded together at the beam-splitting surface. In thisway, both optical paths refract through two flat surfaces at normalincidence, so that no astigmatism is introduced into the beams. Thepupil plane imaging path reflects off of the beam-splitting surface at90 degrees, then travels the length of the rhomboid prism, reflectingoff of the other end at 90 degrees via total internal reflection. Thisbrings the pupil plane imaging path parallel to the field imaging path,but offset by an amount equal to the length of the rhomboid prism. Thisapproach eliminates the need for a coated mirror that must beindependently aligned parallel to the beam-splitter surface, anddramatically reduces the tilt sensitivity of the component.

Adjacent to the beam splitter BS in the field imaging path is arectangular bar of high-index glass. This bar is provided to slow downthe convergence of the light sufficiently, so that the field image andthe pupil image are co-planar and at the desired offset distance (e.g.,3.75 mm). Without this bar, the two images could still be made co-planarif the rhomboid prism were made longer, but this would make it moredifficult for both images to fit onto a small image sensor 113. The barenables the optical axes of the two channels to be within 3.75 mm ofeach other so that both images will fit onto a commercial off-the-shelf(COTS) image sensor 113 whose dimensions are 320 pixel (4.8 mm) wide×256pixel (3.8 mm) The aforementioned bar can be bonded directly to the beamsplitter BS, thus eliminating the need for separate mounting.

Furthermore, the pupil plane imaging path may include another lens PL tocollimate the incoming light and form an image of the entrance pupilonto the image sensor 113. This lens PL may consist of a commercialoff-the-shelf (COTS) lens, e.g., having a diameter of 3 mm and a focallength of 2 mm. According to these parameters, the resultant pupil planeimage can be formed with a width of 315 micron (21 pixels) on the imagesensor, offset by a certain distance (e.g., 3.75 mm) from the imageformed by the field imaging path.

FIG. 32A and FIG. 32B show respective views of a layout of opticalreceivers that, according to the second alternative embodiment, can beimplemented in a crosswind measuring system that is located in front ofa riflescope 8000 mounted to a picatinny rail. These figures assume animplementation of the second alternative embodiment in combination withthe further embodiment described above in connection with FIG. 26 andFIG. 27, which utilizes the symbology projector 200. As shown in FIG.32A and FIG. 32B, just above the 50 mm wind measurement channel 500′ isa 25 mm aperture for the laser transmitter that generates a collimatedbeam to illuminate the target 115. The symbology projector 200 overlaysits images (OAP, confidence metric, etc.) onto the direct view optics ofthe riflescope 8000. Specifically, the overlay display path gets foldedinto the direct view path via a thin beam-splitter plate 105 locateddirectly in front of the riflescope 8000 covering the entire aperture ofthe riflescope 8000. It is advantageous for this beam-splitter plate 105to cover the entire aperture of riflescope 8000 because this helpseliminate the edge diffraction which obscures user's ability to observethe trace of the round fired. The ranging (APD) receiver 400 has forexample a 50 mm collection aperture 104. It is used for ranging to thetarget 115 using time-stamped multiple pulse integration. Except for thesingle pupil plane receiver design described in this second alternativeembodiment, the design of the other three functions, namely the lasertransmitter, electronics associated with the alphanumeric symbologyprojection 200, and the ranging receiver 400 are the same as describedin the first embodiment above.

FIG. 32 b particularly shows the front view of the crosswind and rangemeasurement system, i.e., the XMS system. In the single aperture channel500′, the field imaging path is able to focus the laser illuminated spoton the target 115 down to a couple of pixels, whereas the pupil planeimaging path images the entrance aperture at lens 102 over 21 pixels. Asdescribed earlier, the focused image of the field imaging path is usedfor boresighting as well as to compensate for the environmental factors,whereas the collimated image from the pupil plane imaging path is usedfor crosswind measurements. The field imaging focused laser spot may besensitive to angular beam divergence changes to better than 25micro-radians, whereas the pupil image is insensitive to such changes.

The procedure for boresighting makes use of the electronic crosshair(discussed above in connection with the first embodiment), which issimilar in size to the focused spot D_(T). The electronic crosshair canbe displayed as an overlay on the direct view image which the user seesthrough the scope 8000. The electronic crosshair is moveable in smallincrements via toggle pushbutton switches. During factory alignment, theelectronic crosshair is driven to coincide with the laser spot from adistant target 115, and the position at which the electronic crosshaircoincides with the laser spot is defined as the origin of the electroniccrosshair coordinate system. The size of the focused laser spot D_(T)matters since the electronic crosshair, which is 50 micro-radians, insize can only be co-located with the imaged laser spot on the imagesensor to an accuracy corresponding to the laser spot size D_(T).Therefore, the smaller the imaged laser spot focused on the image sensor113, the better is the boresighting accuracy.

In the field, the boresighting of the system to the riflescope crosshairis accomplished by the user by placing the riflescope crosshairs on adistant target 115, and then driving the electronic crosshair tocoincide with the riflescope crosshairs. This provides the system withthe angular offset from the laser spot axis in both elevation andazimuth. Because the measurements of range and crosswind require thatthe laser beam be on the target 115, the electronic crosshairs are movedto their origin coordinates and used instead of the riflescopecrosshairs. The offset aim point (OAP) that is overlaid on the directview of the riflescope is now referenced to the laser beam spot versusthe crosshair of the riflescope.

Enhancing the Processing Algorithms to Compensate for EnvironmentalFactors:

Based on the field measurements described above, it has been recognizedthat the variables of the crosswind speed V and the refractive indexstructure constant Cn² may impact the laser spot size D_(T). In otherwords, the magnitude of D_(T) is a function of the variables V and Cn²,which can be mathematically represented as D_(T) (Cn², V) and which isgeometrically related to the effective receiver separation D_(eff).

In order to maintain a crosswind measurement accuracy of +/−5% overfield-deployable environmental conditions, the laser spot size D_(T) atthe target 115 may be measured in real time and, if it is different thanthe factory-calibrated size (D₀), the effective receiver separationD_(eff) may be compensated for the change. Such compensations can beperformed in connection with either the two-aperture structure of thefirst embodiment or the single-aperture structure of the secondalternative embodiment.

In the two-aperture design of the first embodiment (where each of thewind measurement channels 500 and 600 has an image sensor 113), thelaser spot size D_(T) can be measured using the same images that areused for measuring the crosswinds (i.e., crosswind measurement signalsP_(A)(t) and P_(B)(t) of FIG. 28).

According to the second alternative embodiment, the real-timemeasurement of the spot size D_(T), at a given range R, can be performedusing the focused image of the laser spot at the image sensor 113, whichis obtained via the field imaging optical path (FIG. 31), as opposed tothe collimated image of the pupil plane.

When the system according to either the first embodiment or the secondalternative embodiment is used in the field, the spot size D_(T) may bemeasured prior to each engagement and compared to the factory-calibratedsize D₀. Factory calibration is preferably done at low Cn² with nearlyzero crosswinds. During factory calibration, the calibrated spot size D₀can be recorded in the non-volatile memory of the system. If the ratioof the D_(T) (Cn², V)/D₀ is different from unity, the effectiveseparation parameter D_(eff) may be compensated using apreviously-determined relationship between D_(off) and D_(T). Thisrelationship can be stored (e.g., as a curve or equation) in anon-volatile memory of the XMS.

The relationship between D_(eff) and D_(T) can be establishedempirically, e.g., by running field tests over different environments toobtain ranges for Cn² and V spanning from low to high values. However, apotential downside of this empirical fit approach is that small errors(less than ±5%) can be introduced in the field measurements if theenvironmental conditions differ somewhat from the measurements used forthe curve fitting in the database.

A more comprehensive approach would be to develop an analytical curvethat relates the changes in laser spot size D_(T) to changes in D_(eff)using the various processing methods. One such example of an analyticalrelationship has been shown in connection with the Peak processingmethod of calculating crosswinds based on a time delay tp (as describedin Wang et al., “Wind measurements by the temporal cross-correlation ofthe optical scintillations,” Applied Optics V20, No. 23, December 1981,the entire contents of which are incorporated herein by reference).Particularly, this analytical relationship has been shown to beapplicable to the Peak method at a time t=0. In summary, it has beenshown that the peak of the time delay tp for a system with a laser spotsize at the target (or emitting source aperture) D_(T), two equalreceiving apertures D_(R), and a constant crosswind V along the path isgiven by:

$\tau_{P}^{- 1} = {V{\int_{0}^{L}\ {{{{{zW}\left( {D_{R},{D_{T}k},z} \right)}}/\rho}{\int_{0}^{L}\ {{{z\left( \frac{z}{L} \right)}}{W\left( {D_{R},D_{T},k,z} \right)}}}}}}$

where k is the wavenumber, L is the range to target, and p is thecenter-to-center separation between the two receiving apertures. Here, Wis a path weighting function for the crosswind speed V(z), and thisfunction W has the form of:

${W\left( {D_{R},D_{T},k,z} \right)} = {\gamma {\int_{0}^{\infty}\ {{{KK}^{- \frac{2}{3}}}{\sin^{2}\left\lbrack {\frac{K^{2}z}{2\; {kL}}\left( {L - z} \right)} \right\rbrack} \times {\left\lbrack \frac{2\; {J_{1}\left( {{KD}_{R}{z/2}\; L} \right.}}{{KD}_{R}{z/2}\; L} \right\rbrack^{2}\left\lbrack \frac{2\; {J_{1}\left( {{{KD}_{T}\left( {1 - \frac{z}{L}} \right)}/2} \right.}}{{{KD}_{T}\left( {1 - \frac{z}{L}} \right)}/2} \right\rbrack}^{2}}}}$

where K is the Kolmogorov atmospheric turbulence spectrum. Dividing bothsides of the above equation by the integrals cancels out the unknownconstant γ. This yields the following geometrical equation relating thelaser spot D_(T) and the effective separation D_(eff) between thereceivers:

$D_{eff} = \frac{\rho {\int_{0}^{L}\ {{{z\left( \frac{z}{L} \right)}}{W\left( {D_{R},D_{T},k,z} \right)}}}}{\int_{0}^{L}\ {{{zW}\left( {D_{R},D_{T},k,z} \right)}}}$

In this above equation, all parameters are known or can be calculated.This analytical approach provides more accurate results in connectionwith the Peak method since environmental conditions that may seldomoccur in field are easier characterized analytically to generate therelationship. However, either approach (empirical or analytical) worksreasonably well to compensate for the environmental changes.

FIG. 33 is a flowchart illustrating an enhanced processing algorithm6000 for measuring the crosswind by compensating for environmentalfactors according to principles described hereinabove. As shown in thisfigure, the algorithm may comprise the following operations:

Operation S6005. The laser spot size D₀ is calibrated, e.g., in a lab,under controlled environmental conditions. Being that this step isintended as a factory calibration, it is shown in FIG. 33 as apre-processing step (i.e., above the dotted line).

Operation S6010. Boresighting is performed in the field by the user.Particularly, the system is boresighted to a target 115 using theelectronic crosshair, as described above. This ensures accurateestimation of the offset aim point (OAP) which is displayed in theshooter's sight.

Operation S6015. The user initiates the crosswind measurement.

Operation S6020. The range to the target L is measured using the rangingreceiver 400 (e.g., APD receiver).

Operation S6025. The time delay (e.g., tp of the Peak method) betweenthe two time-varying signals P_(A)(t) and P_(B)(t) as measured, e.g., bythe single aperture channel 500′ of FIG. 31 based on the pupil planeimaging (alternatively, these signals P_(A)(t) and P_(B)(t) can bemeasured by the respective wind measurement channels 500 and 600 of thefirst embodiment).

Operation S6030. The laser spot size D_(T) is measured. E.g., thismeasurement may be performed using the field imaging path employed inthe single aperture channel 500′ of FIG. 31.

Operation S6035. Here, a determination is made as to whether the ratioof the measured spot size over the calibrated spot size (i.e., D_(T)/D₀)is sufficiently close to unity. If so, no compensation will be necessaryfor the effective separation parameter D_(eff) (as shown in operationS6040), and the algorithm proceeds to S6060.

Operation S6045. This operation is performed if it is determined inS6035 that the ratio D_(T)/D₀ represents a significant deviation fromunity. In this operation, the value for the effective receiverseparation D_(off) is compensated by determining an appropriate valuefor D_(eff) based on the previously stored relationship between D_(eff)and the measured spot size D_(T) obtained by either the empirical oranalytical approach described above.

Operation S6050. In this operation, the crosswind speed V is calculatedusing the appropriate value of D_(eff) as determined in the prioroperations. For instance, assuming that the Peak method is used, thecrosswind speed would be calculated using the equation:

$V = \frac{{Deff}\left( {L,D_{T}} \right)}{t_{p}}$

Operation S6055. After the crosswind is obtained, the results can bedisplayed to the user along with other information (e.g., OAP,confidence metric,) that are derived from the crosswind magnitude V.

In this enhanced algorithm, prior to each engagement of the target 115,boresighting and measurement of laser spot size D_(T) are performed.Boresighting ensures accurate estimation of the offset aim point (OAP),while the spot size D_(T) enables use of correct D_(eff) ensuringaccurate crosswind.

What is claimed is:
 1. A system encased in a housing comprising: anoptical transmitter; a single-aperture optical receiver including: anobjective lens that collects light, at the aperture of the opticalreceiver, which is scattered back from a laser spot illuminated on atarget by the optical transmitter, reimaging optics that generate ade-magnified image of an entrance pupil of the objective lens, and animage sensor that detects the entrance pupil image; a display device;and a processor, wherein: the processor processes the detected image toobtain first and second signals, the first signal representing asummation of pixels from one region of the detected entrance pupil imageas a function of time, the second signal representing a summation ofpixels from the other region of the detected entrance pupil image as afunction of time, the processor processes the first and second signalsto determine a time delay from which a path-weighted average crosswindtoward the target is calculated, and the processor calculates an offsetaim point based on the path-weighted average crosswind, and the offsetaim point is displayed on the display device.
 2. The system of claim 1,wherein each of the first and second signals represents a summation ofpixels from a respective half of the detected entrance pupil image, anda central column of the detected entrance pupil image is filtered out inorder to obtain the first and second signals that are free of crosstalk.3. The system of claim 1, wherein the reimaging optics focuses a portionof the light collected by the objective lens onto the image sensor, thefocused portion of the light being detected by the said image sensor asa field image separate from the detected pupil image.
 4. The system ofclaim 3, wherein the processor uses the detected field image to performa boresighting operation in which a laser beam of the opticaltransmitter is boresighted to a riflescope crosshair by first aligningan electronic crosshair with a laser spot generated by the opticaltransmitter, and then aligning electronic crosshair to the riflescopecrosshair, the processor being able to determine an angular offset foradjusting reference coordinates of the offset aim point based on theboresighting operation.
 5. The system of claim 3, wherein the processoruses the detected field image to measure a size of the laser spotilluminated on the target in real-time, wherein a frame rate used inmeasuring the size of the laser spot is lower than a frame rate appliedto the detection of the entrance pupil image for purposes of obtainingthe first and second signals.
 6. The system of claim 5, wherein theprocessor uses the real-time measurement of the size of the laser spotto compensate the path-weighted average crosswind for environmentalfactors.
 7. The system of claim 6, wherein the processor performs thecompensation of the path-weighted average crosswind by using anempirically determined relationship between the real-time measurement ofthe size of the laser spot and a parameter representing an effectivereceiver separation, said parameter being used to calculate thepath-weighted average crosswind.
 8. The system of claim 6, wherein theprocessor performs the compensation of the path-weighted averagecrosswind by using an analytical equation that relates the real-timemeasurement of the size of the laser spot to a parameter representing aneffective receiver separation, said parameter being used to calculatethe path-weighted average crosswind.
 9. A method employed by a systemencased in a housing, comprising: utilizing an optical transmitter inthe system to illuminate a laser spot on a target; utilizing anobjective lens at an aperture of a single-aperture optical receiver inthe system to collect light which is scattered back from the laser spotilluminated on the target; utilizing reimaging optics in thesingle-aperture optical receiver to generate a de-magnified image of anentrance pupil of the objective lens, and utilizing an image sensor inthe single-aperture optical receiver to detect the entrance pupil image;utilizing a processor in the system to: process the detected image toobtain first and second signals, the first signal representing asummation of pixels from one region of the detected entrance pupil imageas a function of time, the second signal representing a summation ofpixels from the other region of the detected entrance pupil image as afunction of time, process the first and second signals to determine atime delay from which a path-weighted average crosswind toward thetarget is calculated, and calculate an offset aim point based on thepath-weighted average crosswind; and displaying the offset aim point ona display device of the system.
 10. The method of claim 9, wherein eachof the first and second signals are obtained by summing pixels from arespective half of the detected entrance pupil image, and filtering outa central column of the detected entrance pupil image.
 11. The method ofclaim 9, further comprising: utilizing the reimaging optics are utilizedto focus a portion of the light collected by the objective lens onto theimage sensor, the focused portion of the light being detected by thesaid image sensor as a field image separate from the detected pupilimage.
 12. The method of claim 11, further comprising: utilizing theprocessor to process to: perform a boresighting operation based on thedetected field image in which a laser beam of the optical transmitter isboresighted to a riflescope crosshair by first aligning an electroniccrosshair with a laser spot generated by the optical transmitter, andthen aligning electronic crosshair to the riflescope crosshair; anddetermine an angular offset for adjusting reference coordinates of theoffset aim point based on the boresighting operation.
 13. The method ofclaim 11, further comprising: utilizing the processor to measure a sizeof the laser spot illuminated on the target in real-time based on thedetected field image, wherein a frame rate used in measuring the size ofthe laser spot is lower than a frame rate applied to the detection ofthe entrance pupil image for purposes of obtaining the first and secondsignals.
 14. The method of claim 11, further comprising: utilizing theprocessor to compensate the path-weighted average crosswind forenvironmental factors based on the real-time measurement of the size ofthe laser spot.
 15. The method of claim 14, wherein the processorcompensates the path-weighted average crosswind by using an empiricallydetermined relationship between the real-time measurement of the size ofthe laser spot and a parameter representing an effective receiverseparation, said parameter being used to calculate the path-weightedaverage crosswind.
 16. The method of claim 14, wherein the processorcompensates the path-weighted average crosswind by using an analyticalequation that relates the real-time measurement of the size of the laserspot to a parameter representing an effective receiver separation, saidparameter being used to calculate the path-weighted average crosswind.